Add deprecation warning to BackgroundWorker.

I also reworded it, focusing on what to do.

src/lime/system/BackgroundWorker.hx

@@ -1,3 +1,4 @@
 package lime.system;
 
+@:deprecated("Replace references to lime.system.BackgroundWorker with lime.system.ThreadPool. As the API is identical, no other changes are necessary.")
 typedef BackgroundWorker = ThreadPool;