Adds a SpriteWidget and simplifies sample game setup

sky/examples/game/main.dart

 import 'dart:sky';
 
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/raised_button.dart';
 import 'package:sky/widgets/widget.dart';
 
 import 'lib/game_demo.dart';
 import 'lib/sprites.dart';
 
 void main() {
@@ skipping 11 matching lines @@
 
 void allLoaded(ImageMap loader) {
   _loader = loader;
   runApp(new GameDemoApp());
 }
 
 class GameDemoApp extends App {
 
   Widget build() {
     return new Stack([
-      new GameDemo(),
+      new SpriteWidget(new GameDemoWorld(_loader)),
 //      new StackPositionedChild(
 //        new Flex([
 //          new FlexExpandingChild(
 //            new RaisedButton(child:new Text("Hello")),
 //            key: 1
 //          ),
 //          new FlexExpandingChild(
 //            new RaisedButton(child:new Text("Foo!")),
 //            key: 2
 //          )
 //        ]),
 //        right:0.0,
 //        top: 20.0
 //      )
     ]);
   }
 }
 
 ImageMap _loader;
-
-class GameDemo extends OneChildRenderObjectWrapper {
-
-  GameDemo({ Widget child, Object key })
-  : super(child: child, key: key);
-
-  GameDemoBox get root { return super.root; }
-  GameDemoBox createNode() => new GameDemoBox(new GameDemoWorld(_loader));
-}