title: DocumentHeaderTemplate affects title set on LayoutAnchorables
body: <p>I have implemented the solution given in <a href="https://stackoverflow.com/questions/18668705/avalondock2-layoutitemtemplate-only-for-documents">AvalonDock2 : LayoutItemTemplate only for documents</a>. However I have a problem with setting the title on my Documents. I have inserted a DocumentHeaderTemplate</p>
  
  <pre><code>        &lt;xcad:DockingManager.DocumentHeaderTemplate&gt;
              &lt;DataTemplate&gt;
                  &lt;TextBlock Text="{Binding Content.Name}" /&gt;
              &lt;/DataTemplate&gt;
          &lt;/xcad:DockingManager.DocumentHeaderTemplate&gt;
  </code></pre>
  
  <p>This works for my bounded documents, but overrides my Title set on my LayoutAnchorable and gives an empty title when I set this. If I remove the DocumentHeaderTemplate I get my desired title on the anchorables, but not on my documents. My LayoutAnchorable follows:</p>
  
  <pre><code>                 &lt;xcad:LayoutAnchorable ContentId="Log" Title="Log" CanClose="False"&gt;
  </code></pre>