(svn r1768) -Codechange: Store town index in _map2 of town tiles
Moved house type from _map2 to _map3_hi for MP_HOUSE
Moved foundation and roadworks from _map2 to _map3 for
MP_STREET
This increases game speed by a factor of around 15(!) if many cities are around.
Converting an old game is done automagically, but can take a while