Drag&Drop support for child items in QQuickWidget

This is a quick-fix. The real issue is that QWidget and QWindow
reacts differently when a QDragEnterEvent is rejected.

Change-Id: I3e3e50610c14acfaada9c1b243b88cfe9eae54b2
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>