title: Fire event when switching stacks of StackLayoutPanel in GWT 2.0
body: <p>I'm trying to catch an event, when I switch the stacks of a StackLayoutPanel in GWT 2.0.<br>
  The biggest problem is that I don't know which event gets fired and there seems to be no documentation, I have added a ChangeHandler with addDomHandler() but that hasn't worked.<br>
  Unfortunately the StackLayoutPanel doesn't implement the getSelectedIndex()-function so I can't just use an ClickEvent and then check if the selected index has changed.</p>
  
  <p>Is my only solution to use the StackPanel or is there a way to get this to work with the StackLayoutPanel?</p>