title: QGraphicsItem doesn't receive mouse hover events
body: <p>I have a class derived from <code>QGraphicsView</code>, which contains <code>QGraphicsItem</code>-derived elements.  I want these elements to change color whenever the mouse cursor hovers over them, so I implemented <code>hoverEnterEvent</code> (and <code>hoverLeaveEvent</code>):</p>
  
  <pre><code>void MyGraphicsItem::hoverEnterEvent(QGraphicsSceneHoverEvent* event)
  {
      update (boundingRect());
  }
  </code></pre>
  
  <p>However, this event handler code is never executed.  I've explicitly enabled mouse tracking:</p>
  
  <pre><code>MyGraphicsView::MyGraphicsView(MainView *parent) :
      QGraphicsView(parent)
  {
      setMouseTracking(true);
      viewport()-&gt;setMouseTracking(true);
      ...
  }
  </code></pre>
  
  <p>Still, no luck.  What am I doing wrong?</p>