title: WPF binding screentip with relative source
body: <p>I was expecting the title of the screen tip to show "X" as well, but it is empty:</p>
  
  <pre><code>&lt;Fluent:Button x:Name="rbNewProject"
          Header="X"&gt;
  
      &lt;Fluent:Button.ToolTip&gt;
          &lt;Fluent:ScreenTip Title="{Binding Header, RelativeSource={RelativeSource FindAncestor, AncestorType=Fluent:Button}}"&gt;
          &lt;/Fluent:ScreenTip&gt;
      &lt;/Fluent:Button.ToolTip&gt;
  
  &lt;/Fluent:Button&gt;
  </code></pre>
  
  <p>I suspect my binding expression is wrong, but I can't figure it out...</p>