CMS-3765 Replace ExtJS grid - Try move the new TreeGrid to api.ui.treegrid

Removed TreeItem class.
Added `TreeGrid.hasChildren()` method, that has to be overridden in most
cases.
Added generic data id check to TreeNode.
Merge pull request #858 from enonic/CMS-3765

CMS-3765 Replace ExtJS grid - Try move the new TreeGrid to api.ui.treegr...