[Tab Bar] Accept proposed action only ontop of a tab
You cannot drop a file/folder onto the empty area of the tab bar, so show the appropriate "not allowed" cursor.
- Dragged something on the empty area → got a reject cursor now
- Dragged something on a tab → got the usual "plus" cursor
- Dragged something on the empty area and then moved it over to a tab, cursor updated appropriately
- Dragged something on a tab and then moved it to the empty area, cursor updated appropriately