The documentation says we can change window parent to avoid the widget
deleting the window. That didn't work as the widget didn't get the
child-removed event as it wasn't the parent.
This patch instead uses an event filter on the set parent.
Pick-to: 6.3 6.2
Change-Id: I1f61d1832fcf3257722f305beeefd8f1abf1f656
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>