XCB: fix that modal dialogs can go behind other process windows

Task-number: QTBUG-35302
Change-Id: I1ad7a66e530710d5338a15057254360dae676451
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>