Moved part of WorldRendererLwjgl's functionality into new class RenderableWorldImpl 

* This PR does not change functionality nor the WorldRender interface. 
* Its only goal is to move the chunk/mesh updating functionality into a new, separate interface/class pair: RenderableWorld/RenderableWorldImpl. Methods have been moved into the new class, alongside all necessary class and instance variables. Some of these methods in the WorldRendererLwjgl class are now simple pass-through methods, to keep the WorldRenderer interface intact.
* The move reduces the responsibilities of the WorldRenderer to "just" rendering the world, while shifts the responsibility of keeping up to date meshes and helper chunk lists into the new class.

Much work remains to be done and further PRs will follow. This PR consists of 3 commits, I could however squash them together, into a single commit. I just don't know how to do it yet. =) Let me know if this is desirable.