Activate Qt::WidgetWithChildrenShortcut shortcuts in MDI subwindows

If a MDI subwindow is focused and an ancestor has a QShortCut set
with the Qt::WidgetWithChildrenShortcut flag, it should trigger,
as MDI subwindows are not top-level widgets.

Task-number: QTBUG-32788
Change-Id: I7ec76d493b827ae6678209a56015ab6b432c1ed9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>