Adds a SpriteWidget and simplifies sample game setup

sky/examples/game/lib/game_demo_world.dart

 part of game;
 
+const double _steeringThreshold = 0.0;
+const double _steeringMax = 150.0;
+
 // Random generator
 Math.Random _rand = new Math.Random();
 
 const double _gameSizeWidth = 1024.0;
 const double _gameSizeHeight = 1024.0;
 
 const double _shipRadius = 30.0;
 const double _lrgAsteroidRadius = 40.0;
 const double _medAsteroidRadius = 20.0;
 const double _smlAsteroidRadius = 10.0;
@@ skipping 337 matching lines @@
   Laser.withImage(Image img, Ship ship) : super.withImage(img) {
     size = new Size(20.0, 20.0);
     position = ship.position;
     rotation = ship.rotation + 90.0;
     transferMode = TransferMode.plus;
     double rotRadians = convertDegrees2Radians(rotation);
     _movementVector = pointMult(new Point(Math.sin(rotRadians), -Math.cos(rotRadians)), 10.0);
     _movementVector = new Point(_movementVector.x + ship._movementVector[0], _movementVector.y + ship._movementVector[1]);
   }
 
-  bool move() {
+  void move() {
     position = pointAdd(position, _movementVector);
     _frameCount++;
   }
 }
 
 // Background starfield
 
 class StarField extends Node {
   Image _img;
   int _numStars;
@@ skipping 95 matching lines @@
   double dy = a.y - b.y;
   if (dx < 0.0) dx = -dx;
   if (dy < 0.0) dy = -dy;
   if (dx > dy) {
     return dx + dy/2.0;
   }
   else {
     return dy + dx/2.0;
   }
 }