Highlight only when dropping is possible (fixed #303) and fixed persistant highlight issue (fixed #310)
Merge pull request #354 from dinhviethoa/310-303-drag-drop-ui-fixes

Highlight only when dropping is possible (fixed #303)

Added two delegate methods:

- treeViewAllowsDropCells()
- treeViewAllowsDrop() calls 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).