Core: Added Listenable class (Made class Listener system generic)

Former-commit-id: 3baed32d6720c6455f50af51f262292ece9943fa

src/Nazara/Renderer/RenderTexture.cpp

@@ -376,8 +376,8 @@ bool NzRenderTexture::Create(bool lock)
 		onExit.Reset();
 	}
 
-	NotifyParametersChange();
-	NotifySizeChange();
+	Notify(Listener::OnRenderTargetParametersChange);
+	Notify(Listener::OnRenderTargetSizeChange);
 
 	return true;
 }