QColorDialog: don't create widgets if the platform dialog will be used

It will save memory and also prevent bugs where it's assumed that if
the widgets exist, they should be painted, maintained and/or used as a
current value store.

Task-number: QTBUG-31998
Change-Id: I04e3b016018221a405dd7da1fd455e0ede2233fd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>