Timeline will check for the property 'editable' on individual items.
if editable === true
ignore timeline option 'editable'
allow item to be edited
else if editable === false
ignore timeline option 'editable'
do not allow item to be edited
else
use timeline option 'editable'
If two items are selected and one is not editable, only the editable
item is draggable.
This change required the css class .vis-editable be moved from the
item set to the individual item containers to maintain proper styling.