Fix dnd regression c427ba53

Qt starts drag-and-drop on a mouse button press event. Cococa in
this case won't send the matching release event, so we have to
synthesize it here.

Task-number: QTBUG-72417
Change-Id: I645b6a2733c1ea11ac4545cf3405f826af45fa47
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>