QDockWidget: Keep position when undocking.

Initialize undockedGeometry to roughly the current position.

Task-number: QTBUG-31044
Change-Id: I03cbe280d1215bb58ab721b60e29b45359cde76d
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
(cherry picked from qtbase/0d459619a902e547d8c74745453b3bdb8f67a660)