Add and implement interfaces for systems (ISystem, IUpdatableSystem, etc.), move Color to Utils, rename Handlers/Renderers/Backends to System, move input related classes to an Input subfolder.

TestGame/UiLayer.cs

@@ -22,17 +22,17 @@ public class UiLayer : Layer
         _screenContainer.UpdateRect(Vector2.Zero, Scene.Renderer.WindowSize);
     }
 
-    protected override void OnBeginDraw(Renderer renderer)
+    protected override void OnBeginDraw(RenderSystem renderer)
     {
 
     }
 
-    protected override void OnDraw(Renderer renderer)
+    protected override void OnDraw(RenderSystem renderer)
     {
         _screenContainer.Render(renderer);
     }
 
-    protected override void OnEndDraw(Renderer renderer)
+    protected override void OnEndDraw(RenderSystem renderer)
     {
 
     }