Highlight only when dropping is possible (fixed #303) and fixed persista...

- Highlight only when dropping is possible (fixed #303)
Added two delegate methods:
   - treeViewAllowsDropCells()
   - treeViewAllowsDrop()
Call treeViewDropEffect() more often to update the state.

- Fixed persistant highlight issue (fixed #310)

- Misc:
Added uuid to the treeview to be able to match it later.
For now, we're not able to match the treeview all the time because of a bug in drag&drop.
(We cannot fetch the data on dragOver).

@keertip