Refactored chunk state representation into its own class.

+ added class ChunkState
- removed subtype Chunk.State
* changed class Chunk to use the new class ChunkState
* changed some other classes to use the new class ChunkState

* changed protobuf implementation to simplify encoding/decoding of chunk states

- removed default constructor for class Chunk
- removed implementation of interface Externalizable for class Chunk

Note:
From now on, java object serialization for chunks is no longer supported!
Merge remote-tracking branch 'origin/stackable-chunks-2' into monitoring
* adapted calculation of player position for class ChunkMonitorDisplay
Implemented chunk monitoring.

* extended class Chunk to support chunk monitoring
* added some convenience methods for events to class ChunkMonitor
Show monitoring window at startup.