QMainWindow::restoreDockWidget(): Fix positioning of floating docks with native decoration.

Use QWidget::move() to position floating dock widgets.
Add a test for QMainWindow::restoreDockWidget().

Task-number: QTBUG-45780
Change-Id: I945917728a663916e8c225b9d3d2a75fa508d68f
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>