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>
Android: fix GroupBoxStyle top margin

GroupBoxStyle was using checkmark indicator's height even when
the GroupBox was not checkable => too much spacing.

Change-Id: Ic68d8428471b8835cd92f44610a0d8b281bcc734
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>