OLE2 DND: Check if move is supported
Currently OLE2 DND always triggers a move, even if the source only requested a copy operation. Here we add a check to avoid sending back a DELETE to the source if it only requested a copy.
We could add similar checks to other places like IDropTarget::DragEnter or IDropTarget::DragOver, but doesn't really matter all that much. As GTK3 is in LTS, better avoid too many changes.