title: WPF Binding FallbackValue set to Binding
body: <p>Is there a way to have another binding as a fallback value?</p>
  
  <p>I'm trying to do something like this:</p>
  
  <pre><code>&lt;Label Content="{Binding SelectedItem.Name, ElementName=groupTreeView,
                           FallbackValue={Binding RootGroup.Name}}" /&gt;
  </code></pre>
  
  <p>If anyone's got another trick to pull it off, that would be great.</p>