Fixes #723 Views now are notified when they are added or removing.

Terminal.Gui/Core/Responder.cs

@@ -163,6 +163,18 @@ namespace Terminal.Gui {
 			return false;
 		}
 
+		/// <summary>
+		/// Method invoked when a view is added.
+		/// </summary>
+		/// <param name="view">The view added.</param>
+		public virtual void OnAddedView (View view) { }
+
+		/// <summary>
+		/// Method invoked when a view being removing.
+		/// </summary>
+		/// <param name="view">The view being removing.</param>
+		public virtual void OnRemovingView (View view) { }
+
 		/// <summary>
 		/// Method invoked when a view gets focus.
 		/// </summary>