Register ProxyViewContainer children with parent layout.

- Fixes a crash when used in a GridLayout on iOS since the layout needs to be
notified of any layout children changes.
- Adds a _parentChanged hook for all views.

ui/layouts/layout-base.d.ts

@@ -50,6 +50,16 @@
          */
         removeChildren(): void;
 
+        /**
+         * INTERNAL. Used by the layout system.
+         */
+        _registerLayoutChild(child: view.View): void;
+
+        /**
+         * INTERNAL. Used by the layout system.
+         */
+        _unregisterLayoutChild(child: view.View): void;
+
         /**
          * Calls the callback for each child that should be laid out.
          * @param callback The callback
@@ -95,4 +105,4 @@
          */
         paddingTop: number;
     }
-}
+}