Implement changing the number of render threads without restarting Chunky.

[ThatRedox]

• RenderManager now has an optional method to setThreadCount
• Implement changing the number of threads in DefaultRenderManager and RenderWorkerPool
  • Render workers are now stored in an ArrayList, workers are sequentially assigned id's and seeds.
  • Replace RenderWorkerPool.threads with a getThreadCount method.
  • Update interrupt logic and work waiting logic to support killing zombie workers.
• Allow for overwriting the common thread pool to change the number of threads.
• Update advanced tab to support changing the number of threads.

[ThatRedox]

Should probably do a quick test before merging, its been a while.

[leMaik]