Various UI warnings cleanup (#36169)

* Various UI warnings cleanup

* Revert unnecessary change

* Redoing SpriteSystem as it's non-injectable

* Missed one

* Better entity instantiation

* General cleanup of warnings changes

* Wrong class name!

Content.Client/Parallax/ParallaxGenerator.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Threading;
 using System.Collections.Generic;
 using System.Diagnostics;