title: DependencyProperty on ViewModel of UserControl not updated by Binding
body: <p>I have a UserControl which is used as the basis for items in an ItemsControl:</p>
  
  <p>Main Page xaml:</p>
  
  <pre><code>&lt;ItemsControl ItemsSource="{Binding Systems, Mode=TwoWay}"&gt;
      &lt;ItemsControl.ItemTemplate&gt;
          &lt;DataTemplate&gt;
              &lt;local:ServerGroupControl&gt;
                  &lt;local:ServerGroupControl.DataContext&gt;
                      &lt;local:ServerGroupControlViewModel System="{Binding}"/&gt;
                  &lt;/local:ServerGroupControl.DataContext&gt;
              &lt;/local:ServerGroupControl&gt;
          &lt;/DataTemplate&gt;
      &lt;/ItemsControl.ItemTemplate&gt;
  &lt;/ItemsControl&gt;
  </code></pre>
  
  <p>I am trying to set the 'System' property of each ViewModel (such that it can handle the data for the view), but the property is never set!</p>
  
  <p>Here is the dependancy property declaration in the view model class:</p>
  
  <pre><code>    public static readonly DependencyProperty SystemProperty = DependencyProperty.Register(
          "System",
          typeof(ServerGroup),
          typeof(ServerGroupControlViewModel)
      );
  
      public ServerGroup System
      {
          get { return (ServerGroup)GetValue(SystemProperty); }
          set { SetValue(SystemProperty, value); }
      }
  </code></pre>
  
  <p>The property always keeps it's default value. Any ideas on why this setup does not work?</p>