title: Subclassing QGraphicsItemGroup
body: <p>I have system that has classes derived from QGraphicsWidget. I manage derived class objects in layouts on QGraphicsScene. Now I need a compound item that contain two or more QGraphicsWidget in it and also I need to put that item inside my layout. So I choose QGraphicsItemGroup and write I class like this.</p>
  
  <pre><code>class CompositeItem : public QGraphicsItemGroup,public QGraphicsLayoutItem
  {   
     ...  
  };
  </code></pre>
  
  <p>I only implemented sizeHint function again.
  When add CompositeItem instance to layout it does not shown.
  What may cause this? Where I made wrong? </p>