Change WorldServer.allPlayersSleeping to public

In my new dimensions which allow player respawning, sleeping in beds doesn't fast forward time to day; it just sets the spawn point and kicks the player out of bed. This is to do with the way 'setting time' is handled in non-Overworld worlds.

I've fixed this, but it requires me to use reflection to gain access to a private field, and using reflection every world tick is very costly indeed. If the field were public, it would be immensely helpful.