Many changes to support actual entity spawning:
- Entity location and previousLocation are now tracked more correctly.
- GlowItem is now spawnable and uses metadata for storing its item.
- Chunks now track what entities they contain.
- World I/O now loads and saves entities.
- GlowEntity getType() has been left abstract.
- EntityManager's allocation code improved.
- Non-player entities are ticked before players.
More entity improvements:
- Keep teleported, velocityChanged, and better Velocity state.
- Send metadata and velocity changes as part of normal update cycle.
- Added Vector constructor to EntityVelocityMessage.
- Prevent client from assigning velocities to items.
- Implemented setVelocity() on players.