Use the correct parent window for menu popups

The previous fix applied only to the non-platform menu case and was
broken on OS X at least, when having a combobox in a QQuickWidget.

Task-number: QTBUG-39908
Change-Id: I5dde103c2fe33d52a312cc80b36b47b13e72a188
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>