title: How can I bind against the Index of a ListBoxItem
body: <p>I'd like to bind the z index of list box items to their index.</p>
  
  <p>Ideally, we would have</p>
  
  <pre><code>&lt;Style TargetType="{x:Type ListBoxItem}"&gt;
      &lt;Setter Property="Panel.ZIndex"
              Value="{Binding RelativeSource={RelativeSource Self}, Path=-Index}" /&gt;
      &lt;!-- ... --&gt;
  </code></pre>
  
  <p>However, the list box item does not have an index property.</p>
  
  <p>I can think of a number of crazy solutions but nothing simple and elegant.</p>
  
  <p>Any taker?</p>