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
the first custom one can be placed (no selector GUI, coming soon). This
also moves some global variables to {struct GRFFile} and reorders which
actions are processed in what stage, to get it all working together --
(pasky)
