title: How to make group clickable?
body: <p>I've got a group which is composed of rectangle nodes. 
  They are scattered around the group and a lot of the groups background is visible and unfilled by rectangles. </p>
  
  <p>I have a context menu popup when the rectangle is clicked, and I would like a different context menu to show when the group background is clicked.</p>
  
  <p>I tried</p>
  
  <pre><code>Group g = new Group();
  g.setOnMouseClicked((e) -&gt; {
      world.show(g, Side.TOP, 0, 0);
  });
  </code></pre>
  
  <p>But now when I click on a rectangle both the rectangles context menu shows up as well as the groups context menu.</p>
  
  <p>How could I make it so the group menu shows up only when clicking on the background (where a rectangle is not placed)?</p>