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

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.