QtQuick Dialogs: don't use native windows on iOS

Re-applying changes from 560b95b77ac18edd0954a9b8ce9b92a828b51176
in the qtdeclarative repository which were done after beginning to move
the dialogs into qtquickcontrols.

Task-number: QTBUG-35561
Change-Id: I9651be2066e44310afcd4ec9e626583acfec8c8e
Reviewed-by: Liang Qi <liang.qi@digia.com>