Consider virtual screen when determining dock widget visibility.

Task-number: QTBUG-32260

Change-Id: I8b28e3869a6e3b1ed12a311dfa0100979098fc4b
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
(cherry picked from qtbase/6c37fb70d695df001999c78a27ca50d6d2ac6517)