title: Binding to ActualHeight does not work
body: <p>I tried to create a custom control, having a semitransparent rounded background:</p>
  
  <pre><code>&lt;Canvas&gt;
      &lt;TextBlock x:Name="StopText" Text="Some test text"/&gt;
      &lt;Rectangle Fill="SkyBlue"
                 Width="{Binding Source=StopText, Path=ActualHeight}" 
                 Height="20"  
                 RadiusX="5" RadiusY="5" Opacity="0.2"/&gt;
  &lt;/Canvas&gt;
  </code></pre>
  
  <p>The problem is that I can't, probably, bind to the <code>ActualHeight</code>/<code>ActualWidth</code> properties, cause they are not dependency ones.</p>
  
  <p>What to do to mantain the rectangle and textbox of same size?</p>