Allow to drag and drop tabs
[ChangeLog][QtWidgets][Important Behavior Changes] The tabs
for the tabified docks can be moved by the user.
Task-number: QTBUG-1049
Task-number: QTBUG-2295
Task-number: QTBUG-4532
Task-number: QTBUG-18883
Task-number: QTBUG-35148
Change-Id: I7ef9d4987db081654bd5d648e14370b3d381a720
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>