merged develop and fixed the very broken demos

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

@@ -0,0 +1,16 @@
+using Microsoft.Xna.Framework;
+
+namespace MonoGame.Extended.Entities.Systems
+{
+    public interface IUpdateSystem : ISystem
+    {
+        void Update(GameTime gameTime);
+    }
+
+    public abstract class UpdateSystem : IUpdateSystem
+    {
+        public virtual void Dispose() { }
+        public virtual void Initialize(World world) { }
+        public abstract void Update(GameTime gameTime);
+    }
+}