Allow view to be notified

This is in relation to
#593.
I agree with the issue but is a pretty big breaking change. This should allow the correct sequence of activation, activated callbacks.

Author: erik-hooper

src/Caliburn.Micro.Core/Screen.cs

@@ -109,6 +109,7 @@ async Task IActivate.ActivateAsync(CancellationToken cancellationToken)
             Log.Info("Activating {0}.", this);
             await OnActivateAsync(cancellationToken);
             IsActive = true;
+            await OnActivatedAsync(cancellationToken);
 
             await (Activated?.InvokeAllAsync(this, new ActivationEventArgs
             {
@@ -185,6 +186,15 @@ protected virtual Task OnActivateAsync(CancellationToken cancellationToken)
             return Task.FromResult(true);
         }
 
+
+        /// <summary>
+        /// Called when view has been activated.
+        /// </summary>
+        protected virtual Task OnActivatedAsync(CancellationToken cancellationToken)
+        {
+            return Task.FromResult(true);
+        }
+
         /// <summary>
         /// Called when deactivating.
         /// </summary>