removed the old ecs and refactored the platformer demo to use the new ecs

Source/MonoGame.Extended.Entities/Systems/EntityUpdateSystem.cs

@@ -20,9 +20,16 @@ namespace MonoGame.Extended.Entities.Systems
             {
                 _world = value;
                 _subscription = new EntitySubscription(_world.EntityManager, _aspectBuilder.Build(_world.ComponentManager));
+
+                // TODO: Undisposed events.
+                _world.EntityManager.EntityAdded += (sender, entityId) => OnEntityAdded(entityId);
+                _world.EntityManager.EntityRemoved += (sender, entityId) => OnEntityRemoved(entityId);
             }
         }
 
+        protected virtual void OnEntityAdded(int entityId) { }
+        protected virtual void OnEntityRemoved(int entityId) { }
+
         public Bag<int> ActiveEntities => _subscription.ActiveEntities;
 
         public abstract void Initialize(IComponentMapperService mapperService);