title: Implementing "Expand/Collapse all" with DisclosurePanels in GWT
body: <p>I have a lot of DisclosurePanels nested one inside another (or may not be nested either). I want to implement "expand all" and "collapse all" buttons. 
  Please suggest a clean way to do so.</p>
  
  <p>Note: I tried looking a bit onto Event handling in GWT but got a bit confused with things. I am not sure if I could use Custom Events for my case. It seems that I can define a custom event and its handler and do the addHandler() stuff but in the class that'll implement the handler interface I don't have the instance of DisclosurePanel that'll allow me to use setOpen() property of disclosure panel.</p>