title: GWT image centered over the application
body: <p>I'm wondering about how it works to center an image over the hole GWT application 
  when pressing a button and make it disappear when a special position in the 
  code is reached.</p>
  
  <p>It will be a <code>"waiting.gif"</code> that shows a circle spinning around while the program
  is calculating something. When the result is calculated the GIF should disappear.</p>
  
  <p>Do i have to do it with some kind of pop-up-panel? or how does it work?</p>