Fix position of drop menu on Wayland

On Wayland there is no global cursor position and QCursor::pos() is not
100% reliable. In particular it doesn't work from dropEvent() functions,
where returns the position where the drag started, rather than where the
drop happened. We can use instead the relative position from the
QDropEvent after converting it to global position.

Change-Id: Id3a72bb8e946198039bb97be46793a89c1ef6510