title: Showing a popup menu on QGraphicsScene click or right click
body: <p>Is there a way to show a popup when the user right clicks on an empty portion of the scene?</p>
  
  <p>I'm new at Qt and I've tried slots and subclassing, but to no avail.</p>
  
  <p>No such slot and, respectively:</p>
  
  <blockquote>
    <p>"error: 'QMouseEvent' has not been declared"</p>
  </blockquote>
  
  <p>when trying to implement the <code>onMouseRelease</code> event.</p>