title: Javafx click on a Circle and get it's reference
body: <p>I have a set of Nodes, Circles, on the stage.
  I want to be able to click on one of them and 'select it' (just get a reference to it so I can move it around, change color etc.)</p>
  
  <pre><code>    Pane root = new Pane();
      root.getChildren().addAll( /* an array of Circle objects */ );
  
      Scene scene = new Scene(root, 500, 500, BACKGROUND_COLOR);
  
      scene.setOnMouseClicked(new EventHandler&lt;MouseEvent&gt;() {
          @Override
          public void handle(MouseEvent mouseEvent) {
              // how do I get which Circle I clicked on?
          }
      });
  
      stage.setTitle(TITLE);
      stage.setScene(scene);
      stage.show();
  </code></pre>