make CDockWidget saveState virtual

Author: Wing-summer

src/DockWidget.h

@@ -103,7 +103,7 @@ private Q_SLOTS:
     /**
      * Saves the state into the given stream
      */
-    void saveState(QXmlStreamWriter& Stream) const;
+    virtual void saveState(QXmlStreamWriter& Stream) const;
 
     /**
      * This is a helper function for the dock manager to flag this widget