Fix QBackingStore object leaking

Check q->isTopLevel() before calling setBackingStore() in
QWidgetPrivate::create_sys() to prevent QBackingStore leaking. This is
because QWidget::setBackingStore() will return if isTopLevel() is true and
the newly created QBackingStore object will be leaked.

Change-Id: I2777acd4c317d5019f5b266feae005042026b8be
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>