title: ObservableCollection<T> CollectionChanged event
body: <p>I have an observable collection and I have a collectionChanged event wired up on that.
  I have its items bound to a listbox in the UI.
  When the user removes some items in the UI from the listbox, the CollectioChanged gets fired properly, however, I need to know the indices of the items that were removed.
  The problem is I am not able to an indexOf on the collection after it is changed because it does not have the removed item anymore..</p>
  
  <p>Can we get access to the list of indexes that were removed from an ObservableCollection from the collectionchanged event?</p>