CMS-3598 Double-clicking in ContentGrid2 row should open Content for edi...

...t

Implemented doubleClick event handler for edit.
Renamed module `api.app.browse.grid2` to `api.app.browse.treegrid`.
Renamed `GridPanel2` to `TreeGrid`.
Moved `GridActions` into `api.app.browse.treegrid.TreeGridActions`.
Moved `GridToolbar` into `api.app.browse.treegrid.TreeGridToolbar`.
Removed `grid-panel2.less` and placed its styles into `tree-grid.less`.