CMS-3706 Replace ExtJS grid - Make ContentGridPanel2 behave like old grid when user is searching in ContentBrowseFilterPanel

Added unique id generation in `TreeNode` constructor.
Added filter events handling.
Fixed `SplitPanel` separator bug (separation had invalid position on
render).
Merge pull request #816 from enonic/CMS-3706

CMS-3706 Replace ExtJS grid - Make ContentGridPanel2 behave like old gri...