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.

* self reference edge should now appear in all cases
* add checks for shape width being NaN to get correct circle data
* reorder resize arguments in Diamond, Dot and Star

* element characteristic changes
* assume edge color inheritance is correct before choosing
* Adjust nodes’ chosen’s boolean -> bool
* Need to get user-reset size
* make example edges thicker for more noticeable shadow
* preemptive ES6 fix (see #2500/#2501)
* documentation for the feature that was previously overwritten

Signed-off-by: André Martins <aanm90@gmail.com>

see #1608

- Fixed #1528, #1278, avoided ID's being cast to string for methods that return ID's as well as storePositions casting to string.