Delegate xp_algorithm to system

This patch moves the xp algorithm from a field on the character, to a
delegation to the system.
Renames System to GameSystem

The model `System` is ok, while calling `<anything>.system` raises an
exception, that `system` is a private method. This patch renames the
System model to GameSystem, which eliminates the problem. Further more,
the concept "Game system" is more descriptive of what the model actually
contains.