title: Handle callback for ENTER key in GWT-Ext's NumberField
body: <p>I use GWT-Ext library for building GUI for a webapp.
  I'd like to process an ENTER key press inside of the NumberField.</p>
  
  <p>It should be like this:</p>
  
  <pre><code>    final NumberField requiredHeight = new NumberField();
      requiredHeight.setValue(125);
      requiredHeight.setAllowBlank(false);
      requiredHeight.setAllowNegative(false);
      requiredHeight.setAllowDecimals(false);
      KeyListener listener = new KeyListener() {
  
          public void keyPressed(KeyEvent e) {
              if (e.getKeyCode()==13)
                  addPortlet(requiredHeight.getValue().intValue());
          }
  
      };
      requiredHeight.addKeyListener(listener);
  </code></pre>
  
  <p>But the code doesn't do anything!
  What am I doing wrong? Generally speaking, what is the best way to add such a handler to the field?</p>