Many changes/extensions for monitoring

- removed specialized single thread monitor classes
+ added null thread monitor
* changed all users of thread monitors to use the new SingleThreadMonitor interface

+ added chunk monitor

+ implemented gui for monitoring

* changed handling of tasks in TerasologyEngine so monitoring is supported

Stackable Chunks

* added missing support for stackable chunks in methods Chunk.getAABB() and Chunk.getSubMeshAABB()
More work on monitoring

* extended ChunkMonitor
* changed constructors of class Chunk so the chunks position is always set as early as possible
+ added class WeakChunk
+ added class ChunkMonitorDisplay
+ added class ChunkMonitorPanel
* renamed class ThreadMonitor to ThreadMonitorPanel to avoid confusion with another class named ThreadMonitor

Other stuff

- removed some constructors of class Chunk
- removed object serialization support for class Chunk

Bugfix for LocalChunkProvider

* fixed a bug where chunks get generated multiple times for the same position