title: OnExpanded event for any item in a treeview
body: <p>I'd like to get an event for any expansion of a treeviewitem in my treeview. </p>
  
  <p>The reason for this, a bit unrelated to the original question:
  I am creating a tree that relates closely to an xml file tree, but I am allowing an include element  in the xml so the tree can go across multiple files. I'd like to set the itemssource property of treeviewitems within my treeview upon expansion.</p>