Move SnapToDefaultButton from QPlatformDialogHelper to QPlatformTheme

Since QPlatformTheme covers all dialogs whereas QPlatformDialogHelper is
really only for the native dialogs then the SnapToDefaultButton hint is
moved as it has relevance for all dialogs

Task-number: QTBUG-32631

Change-Id: I1dce0bb4abcd4cfd39c4a199a33fc7078176ab4b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>