title: OnInitialize and OnActivate are not called on child View Models
body: <p>I expected that child View Models inheriting from Screen would participate in the Parent Screen's life-cycle. However, it appears this is not the case. For example:</p>
  
  <pre><code>public class ParentViewModel : Screen
  {
      public ChildViewModel Child { get; set; }
  
      public ParentViewModel(ChildViewModel childViewModel)
      {
          this.Child = childViewModel;
      }
  
      public override void OnInitialize() { // called - as expected }
  
      public override void OnActivate() { // called - as expected }
  
      public override void OnDeactivate() { // called - as expected }
  }
  
  public class ChildViewModel : Screen
  {
      public override void OnInitialize() { // not called - why? }
  
      public override void OnActivate() { // not called - why? }
  
      public override void OnDeactivate() { // not called - why? }
  }
  </code></pre>
  
  <p>Is it possible to have a child Screen participate in the parent Screen's life-cycle?</p>