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

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

@@ -1,13 +1,16 @@
-using Microsoft.Xna.Framework;
+using System;
+using Microsoft.Xna.Framework;
 
 namespace MonoGame.Extended.Entities.Systems
 {
-    public abstract class UpdateSystem
+    public abstract class UpdateSystem : IDisposable
     {
         protected UpdateSystem()
         {
         }
 
+        public virtual void Dispose() { }
+
         public abstract void Update(GameTime gameTime);
     }
 }