title: How to catch AvalonDock undocking and Docking events?
body: <p>I am Using AvalonDock in a WPF project.
  And I would like to konw how to handle the Docking and Undocking event.
  i have tried MouseDown and Up while Dragging but no apparent success.</p>
  
  <p>...</p>
  
  <pre><code>&lt;ad:DockingManager x:Name="dockManager" Grid.Row="2" Margin="0,25,0,0" Background="Transparent" &gt;
                              &lt;ad:ResizingPanel Orientation="Vertical" &gt;
                                  &lt;ad:ResizingPanel Orientation="Horizontal" &gt;
                                      &lt;ad:DocumentPane Background="Transparent"&gt;
                                          &lt;ad:DocumentContent Title="Document View!"&gt;
                                              &lt;Grid Background="Transparent"&gt;
                                              &lt;Rectangle Name="rect_Zoom" Grid.Row="2" MouseLeftButtonDown="rect_Zoom_MouseDown" MouseMove="rect_Zoom_MouseMove" MouseWheel="rect_Zoom_MouseWheel" MouseUp="rect_Zoom_MouseUp"  /&gt;
  </code></pre>
  
  <p>...                                            </p>