CMS-4182 TreeGrid - Automatic loading of remaining children

Created a separate styles for the `CompareContentGrid`.
Added the generation of the id's for the  empty nodes.
Removed `TreeGrid.canvasElement` property.
Simplified and optimized the DOM selection for the animation and
postload in `TreeGrid`.
Added `Element.elementsFromRequest()` static method to get the array of
objects from string.
Merge pull request #988 from enonic/CMS-4182

CMS-4182 TreeGrid - Automatic loading of remaining children