Refactor model wrapper subscription

The model wrapper should not know about the server but only subscribe to it.
This fixes this behaviour and the code using the model wrapper accordingly.
Revise relation between player and level

The special presence object was not a good idea. Now the player has a level, in
which they are, while the level has no knowledge of the players. At some point
this should be taken care of by some kind of querying mechanism.
Implement simple model IDs

The model objects within the application require some form of ID to uniquely
identify them within the context of a common game across a heterogenous
server-client architecture. Therefore a mechanism to provide and maintain these
IDs has been implemented here.
Implement a simple loader handling model objects

The loader is supposed to provide model objects that are available on different
resources (memory, disk, remote server). So far the interface does only
implement a storage dictionary of weakrefs.
Add test directory

This is meant for more complex unit tests and integration tests.
Test persistence of an example model

This is a preliminary test of the implicit creation of objects as weak and lazy
references.