title: How to catch user leaving a page and cancelling it
body: <p>When a user leaves the GWT app, I would like to open a confirm dialog and offer them the choice to stay, i.e. Confirm("are you sure you want to leave this page", "yes", "no").</p>
  
  <p>I know how to build the dialbox. :)</p>
  
  <p>The question is, how to I catch the event of a user leaving the page and how to I cancel it?</p>
  
  <p>Daniel</p>