#18 Memory leaks

[Wolfgang Pausch]

Consider starting a game (from startup-window) and aborting to
startup-window (currently only in cvs possible). This leads into a
memory leak affecting at least quadmap.

The reason probably is that QuadMap isn´t removed from
GameChangeManager in that case.

TODO: Introduce a safe way of removing listener-relationsships
from GameChangeManager when destructing objects. Possibly
using finalize.

(maybe GameChangeManager should be destructed at all when
aborting to startup-window? But this wouldn´t fix the problem for
objects being destructed in game)