Adds better explosions, shield, and game over

sky/sdk/example/game/lib/action.dart

 part of sprites;
 
-typedef void PointSetter(Point point);
+typedef void ActionCallback();
 
 abstract class Action {
   bool _finished = false;
 
   void step(double dt);
   void update(double t) {
   }
 
   double get duration => 0.0;
 }
@@ skipping 102 matching lines @@
 
   void update(double t) {
     fire();
     _finished = true;
   }
 
   void fire();
 }
 
 class ActionCallFunction extends ActionInstant {
-  Function _function;
+  ActionCallback _function;
 
   ActionCallFunction(this._function);
 
   void fire() {
     _function();
   }
 }
 
-class ActionRemoveFromParent extends ActionInstant {
+class ActionRemoveNode extends ActionInstant {
   Node _node;
 
-  ActionRemoveFromParent(this._node);
+  ActionRemoveNode(this._node);
 
   void fire() {
     _node.removeFromParent();
   }
 }
 
 class ActionRepeatForever extends Action {
   final ActionInterval action;
   double _elapsedInAction = 0.0;
 
@@ skipping 77 matching lines @@
     for (int i = _actions.length - 1; i >= 0; i--) {
       Action action = _actions[i];
       action.step(dt);
 
       if (action._finished) {
         _actions.removeAt(i);
       }
     }
   }
 }