Enable input methods for QQuickWidget

Make it possible for widgets to have non-widget focus objects. Since
we cannot add new virtual functions due to binary compatibility, we
have to do it in QWidgetPrivate.

Task-number: QTBUG-42677
Change-Id: I5f74daed2793c1c149bbe02e54ff2f7e2ad1af9e
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>