Fixes a race condition that set an item's group to be set to undefined
if it was dropped and then dragged again without a redraw happening in
between.

The returned node id's were exactly the wrong way around;
'to' returned the parents, 'from' returned the children.

This fix swaps the tests for determining which to return.

* Fix for timeline TimeStep.roundToMinor - resolving issue #3105

* Adding some tests for TimeStep

Code in the `_line()` of the sibling Bezier edge types was either extremely similar or identical.
The Bezier drawing code has been consolidated in a single method in the parent class.

This method is needlessly complicated. Code has been adjusted to better show the intent.

In addition:
- reuse of variables already present, notably `dx` and `dy`
- put recurring code fragments into local variables
- removed second conditions in constructs of the form `if (condition) ... else if (!conditioni) ...`

The refactoring can be taken further, but it would change the conditional flow, which would complicate reviewing.
The current changes highlight the similarities between the code blocks.

* Consolidate code for determining the pixel ratio

* - Removed local param  'pixerRatio' from CanvasRenderer. Now only canvas.pixelRatio is used.
- consolidated ctx.transform() calls in Canvas
- Added/edited commenting (also TODO's)
- Added Canvas.getContext()

Fix for #3036

Option `smooth.type: dynamic` is not allowed for hierarchical layouts.
This was handled properly for the main options, but not for the node-specific options.

Options within node instances arei now checked for `smooth.type: dynamic` and replaced
by `horizontal` or `vertical`. The implementation adds listener `_adjustEdgesForHierarchicalLayout`
in `LayoutHandler`. This listener must be activated every time noder- specific options might change.
This happens in the logical places within `EdgeHandler`.

* clusters in hierarchical working for sortorder 'hubsize'

* Code cleanup

* Protect Network from zero and negative mass values

Fix for #3133

Option-field 'node.mass` must be >= 0.
Checks have been added at the nodes level, for both nodes-global and nodes specific options.
In addition, an internal check has been added for `NodeHandler.defaultOptions`.

The documentation has been adjusted for this change.

* Fix whitespace

* Prevent crashes from invalid id's in `Clustering.findNode()`

Fix for #3163

- Added safeguards in said method, to prevent exceptions happening when invalid id's are passed in.
- Adjusted documentation for said method
- Added notes to flag unused methods in `Clustering.js`

* Removed incorrect NOTE-comments from methods that are indeed used

Fix for #3106

Event `stabilizationProgress` is now always sent for iteration 0 and for the very last stabilization iteration.

* First working version of new class DataGroup.

* Adjustments for review - done all points except last

* DRY distinct values, clean up sort code

* Added missing @param's to comments in DataGroup

Call on issue brought to light by #2990.

`ctx` should not be passed as a parameter to the call - not needed and not conform to documentation.
Also cleaned up the calling code to remove the double `fontOptions.chooser()` call.

* Properly handle newline escape sequences in strings for DOT

* Adjust comments in code

* First save after refactoring

* Fixes after testing

* Final changes

Currently `align` is only available as a global timeline configuration
option. This change allows overriding alignment on a per-item basis.

* Fix selection nodes so that option selectConnectedEdges works as expected

* Using instance var select instead of local var in Edge.drawLabel()

* Changed order of (de)select events for network

* Fixed parameters to _generateClickEvent() to be compatible with previous.

* Added param 'direction' to Network.getConnectedNodes()

* Redo commit - Network.js does not need to change

Awesome work! Thanks again!

* Add support for multiple class names in utils add/remove class methods.
Reset subgroup heights in resetSubgroups method.

* Adjust to better process class names

* Add 'showTooltips' timeline option

* Only show timeline popup if option showTooltips is true

* Add 'showTooltips' option to timeline docs

* Add tooltips disabled timeline example

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* Improve redraw performance by not restacking non-visible groups (related to #2835)

* only restack necessary groups on redraw (related to #2835)

* replacing all ES6 imports with CJS require calls
resolves #2934
used the following regex to apply the changes in lib:
s/import\s+(\w+)\s+from\s+(.*);\s*$/var $1 = require($2).default;/
s/import\s+(\w+)\s+from\s+(.*)\s*$/var $1 = require($2).default;/

* cleaning up inconsistencies

* Add warnings and errors for image loading

* Fixes and commenting for image loading

* Fix typo

* Small fixes due to review

Sorry for the delay on this... It took me a while to confirm this PR again. I don;t know why I didn't merge it at the time I reviewed it. Looks great to me!

* mouseup and mousedown events

* Fix error

* CachedImage with preredendered zoom images; first working version.

* Fixes for missing images and usage brokenImage.

* Remove unused method

* Added height member of CachedImage, note about svg's

* Refactor propagateFonts(), first working version. Default font  for 'mono' is now 'monospace'

* Tested and cleaned up refactoring changes

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* Fix item click and doubleclick

* Remove new empty line

* Return id of item tapped

* Fix comments from Review

* Add missing semicolon

* Add Gitter badge (#2179)

* do not generate source-maps in distribution version

* add 'dist' folder for deployment

* added Badges

* added codeclimate badge

* added @Tooa to the support team

* added badges from isitmaintained.com (#2517)

* do not ignore dist and test folders in master

* generated dist files for v4.18.0

* generated dist files for v4.18.1

* Cheap fix for bug #2795

* Update to PR #2826 to use newline format

* changed to v4.18.1-SNAPSHOT

* chore(docs): general improvements (#2652)

* removed NOTICE file

* updated license date range to include 2017

* chore(docs): updated support team members

* chore: updated dependencies and devDependencies (#2649)

* Fixes instanceof Object statements for objects from other windows and iFrames. (#2631)

* Replaces instanceof Object checks with typeof to prevent cross tab issues.

* Adds missing space.

* chore: removed google-analytics from all examples (#2670)

* chore(docs): Add note that PRs should be submitted against the `develop` branch (#2623)

Related to: https://github.com/almende/vis/pull/2618
Related to: https://github.com/almende/vis/pull/2620

* feat(timeline): Change setCustomTimeTitle title parameter to be a string or a function (#2611)

* change setCustomTimeTitle title parameter, Now could be an string or a function
* Fixed indent and spacing

* feat(timeline): refactor tooltip to only use one dom-element (#2662)

* feat(network): Allow for image nodes to have a selected or broken image (#2601)

* feat(tests): run mocha tests in travis ci (#2687)

* Added showX(YZ)Axis options to Graph3d (#2686)

* Added showX(YZ)Axis to Graph3d

* Added show_Axis options to docs and playground example

* Resolved merge conflict

* Added show_Axis options to docs and playground example

* fix(build): use babel version compatible with webpack@1.14 (#2693)

fixes #2685

* feat(docs): use babel preset2015 for custom builds (#2678)

* add link to a mentioned example (#2709)

* chore(lint): added support for eslint (#2695)

* Trivial typo fix in how_to_help doc. (#2714)

* fix(timeline): #2598 Flickering onUpdateTimeTooltip (#2702)

* Fix redraw order
* Fix error when option is not defined
* Allow template labels
* Add .travis.yml file
* Add experiment travis code
* Fix react example
* Add animation to onUpdateTooltip

* fix(timeline): #778 Tooltip does not work with background items in timeline (#2703)

* Fix redraw order
* Fix error when option is not defined
* Allow template labels
* Add .travis.yml file
* Add experiment travis code
* Fix react example
* Make items z-index default to 1

* Add initial tests for Timeline PointItem (#2716)

* fix(timeline): #2679 TypeError: Cannot read property 'hasOwnProperty' of null (#2735)

* Fix redraw order
* Fix error when option is not defined
* Allow template labels
* Add .travis.yml file
* Add experiment travis code
* Fix react example
* Fix bug in item editable

* feat(timeline): #2647 Dynamic rolling mode option (#2705)

* Fix redraw order
* Fix error when option is not defined
* Allow template labels
* Add .travis.yml file
* Add experiment travis code
* Fix react example
* Add toggleRollingMode option
* Update docs with toggleRollingMode option

* fixes timestep next issue (#2732)

* feat(timeline): added new locales for french and espanol (#2723)

* Fix eslint problem on Travis. (#2744)

* fix: Range.js "event" is undeclared (#2749)

* fix(timeline): #2720 Problems with option editable (#2743)

Clean up and centralise edit status for Timeline Items.

* feat(network): Improve the performance of the network layout engine (#2729)

* Improve the performance of the network layout engine

Short-cut the execution of a number of methods in LayoutEngine to make them
handle highly-connected graphs better.

* Demonstrations of layouts of large networks

* Added support to supply an end to bar charts to have them scale (#2760)

* Added support to supply an X2 to bar charts to have them scale

* Fixed graph2d stacking issue.  It no longer takes into account hidden items

* Changed x2 to end per recommendation and added this to the docs

* Initial tests for timeline ItemSet. (#2750)

Somewhat more complicated setup, associated with the need for a real window.

* [Timeline] Modify redraw logic to treat scroll as needing restack. (#2774)

This addresses #1982 and #1417.

It possibly reduces performance, but correctness seems better.

* fix(timeline): #2672 Item events original event (#2704)

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* Fix events returned from mouse events

* Fix example

* Rename censor to stringifyObject in example

* [timeline] Update examples to use ISOString format. (#2791)

Resolves #2790

* [timeline] Update serialization example to use ISOString dates. (#2789)

Resolves #2696

* added github templates for issues and pull-requests (#2787)

fixes #2418

* feat(timeline): Add item data as argument to the template function (#2799)

* Fix regression introduced in #2743. (#2796)

* Fix for issue #2536 (#2803)

* Fix for issue #2536

* Adjusted documentation for fix.

* Adjustments due to review

* Grrrrr whitespace

* Fixed Travis issue

* Cheap fix for bug #2795

* Update to PR #2826 to use newline format

* Update to gitignore to reflect changes on remote

* clean dist folder

* Local gitignore update

* Just a first example file for the week scale feature

* Allowing sourcemap creation

* Initial (non-functional) commit to ensure we insert code at the right places (check TODOs)

* Functional, not bug-free version which works with locale aware week numbers.

* Locale-aware implementation and simplified major labels to a full year

* Trying to make the major labels show the correct start date

* Working implementation of week numbers using localization.

* removing development leftovers

* Updated package.json

* Reintagrate package.json from accidental deletion

* Updates for package.json

* Fixing package.json

* Integrate the week numbers feature in the documentation.

* Reverting local changes to .gitignore

* Reverting local changes

* Extending examples to cover the case when 1st day of week and 1st day of month align; Fixing display bug so that week numbers are not repeated in minorLabels

* Putting the examples into a loop

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* Add pausing of rangechanged in scroll

Add subgroup updates to Item.setData.
Remove group updating logic from ItemSet._updateItem.
Fixes #2939

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* Fix adding items with double click

* Fix redraw order

* Fix error when option is not defined

* Allow template labels

* Add .travis.yml file

* Add experiment travis code

* Fix react example

* Make selectiveExtend accept null/undefined parameters