big refactoring of level and world class. A level is now basically a set of

big refactoring of level and world class. A level is now basically a set of
sectors (or sublevels). The Sector class has been merged with the old world
class. Also I've rewritten some parts of the load/save code and changed the
fileformat a bit to support several sectors.
On the way I fixed some bugs (and maybe introduce new ones...)

SVN-Revision: 1371