title: Get internal item no from ListCollectionView
body: <p>I've extended a ListCollectionView and overridded GetItemAt like this:</p>
  
  <pre><code>public class LazyLoadListCollectionView : ListCollectionView
  {
  
    public override object GetItemAt(int index)
    {
      object rc = base.GetItemAt(index);
      // do something
      return rc;
    }
  }
  </code></pre>
  
  <p>Now for my "do something" I need the position of the item in the internal list. As long as the ListCollectionView is not sorted "index" of the ListCollectionView will be the same for the internal collection, but as soon as the ListCollectionView is resorted index will match the index in the internal collection (the internal collection being an ObservableCollection).</p>
  
  <p>So where does ListCollectionView get the internal collections index from the index in the ListCollectionView? Shouldn't there be a "int ConvertToInternalIndex(int index)" somewhere?</p>