title: Right-align items within wpf expander header
body: <p>I want to have some text in an expander header left aligned, then some text right aligned.
  I have found how to expand the header to the width of the container, and thought I could simply add a dockpanel and set the second text block to Dock Right, but it doesn't seem to help. Any solutions?</p>
  
  <pre><code>&lt;Expander&gt;
    &lt;Expander.Header&gt;
      &lt;DockPanel
        Width="{Binding
          RelativeSource={RelativeSource
          Mode=FindAncestor,
          AncestorType={x:Type Expander}},
          Path=ActualWidth}"&gt;
        &lt;TextBlock
          Text="I am header text…"
          Background="LightBlue"
        /&gt;
        &lt;TextBlock DockPanel.Dock="Right"
          Text="I am header text…"
          Background="Yellow"
        /&gt;
      &lt;/DockPanel&gt;
    &lt;/Expander.Header&gt;
  &lt;/Expander&gt;
  </code></pre>