Dialogs: don't use widgets on mobile touch-based platforms

It doesn't make sense to use widget-based dialogs in a QtQuick app
on a mobile platform because widgets are not optimized for touch.

Change-Id: I8659f247eb6e75ca827e8b42a0fb1e7f65dceee1
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>