Only do a repolish() of the widget if it was already polished

By ensuring that the widget is already polished before doing a repolish it
means that if you reset the stylesheet then it unsets any changes that
the stylesheet had applied to the widget.

Task-number: QTBUG-18958

Change-Id: Ie0aeda0dac9f2211b7feca138c115cf2b48aac80
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/widgets/kernel/qwidget.cpp

@@ -2404,7 +2404,8 @@ void QWidget::setStyleSheet(const QString& styleSheet)
     }
 
     if (proxy) { // style sheet update
-        proxy->repolish(this);
+        if (d->polished)
+            proxy->repolish(this);
         return;
     }