Adds (somewhat crappy) controls to example game

sky/examples/game/lib/game_box.dart

+part of game;
+
+const double _steeringThreshold = 20.0;
+const double _steeringMax = 50.0;
+
+class GameBox extends SpriteBox {
+
+  GameBox(GameWorld game) : super(game, SpriteBoxTransformMode.letterbox);
+
+  GameWorld get _gameWorld => this.rootNode;
+
+  // Handle pointers
+  int _firstPointer = -1;
+  int _secondPointer = -1;
+  Vector2 _firstPointerDownPos;
+
+  void handleEvent(Event event) {
+    if (event is PointerEvent) {
+      Vector2 pointerPos = new Vector2(event.x, event.y);
+      int pointer = event.pointer;
+
+      switch (event.type) {
+        case 'pointerdown':
+          if (_firstPointer == -1) {
+            // Assign the first pointer
+            _firstPointer = pointer;
+            _firstPointerDownPos = pointerPos;
+          }
+          else if (_secondPointer == -1) {
+            // Assign second pointer
+            _secondPointer = pointer;
+            _gameWorld.controlThrust(1.0);
+          }
+          else {
+            // There is a pointer used for steering, let's fire instead
+            _gameWorld.controlFire();
+          }
+          break;
+        case 'pointermove':
+          if (pointer == _firstPointer) {
+            // Handle turning control
+            double deltaX = pointerPos[0] - _firstPointerDownPos[0];
+            if (deltaX > _steeringThreshold || deltaX < -_steeringThreshold) {
+              double turnForce = (deltaX - _steeringThreshold)/(_steeringMax - _steeringThreshold);
+              if (turnForce > 1.0) turnForce = 1.0;
+              if (turnForce < -1.0) turnForce = -1.0;
+              _gameWorld.controlSteering(turnForce);
+              print("steering: $turnForce");
+            }
+          }
+          break;
+        case 'pointerup':
+        case 'pointercancel':
+          if (pointer == _firstPointer) {
+            // Un-assign the first pointer
+            _firstPointer = -1;
+            _firstPointerDownPos = null;
+            _gameWorld.controlSteering(null);
+          }
+          else if (pointer == _secondPointer) {
+            _secondPointer = -1;
+            _gameWorld.controlThrust(null);
+          }
+          break;
+        default:
+          break;
+      }
+    }
+  }
+
+}