split off terrain data caches from the gamemap object

This commit splits some of the functionality of the gamemap object
into a new class, the "terrain_type_data" object. This class is
intended to hold a record of known terrains and the terrain code ->
terrain type map. It handles merging in new terrain definitions,
and it is shared between map objects. It is also cached by the game
config manager, so that we only recalculate this data when we
change configs.

There is in fact a *semantic* change going on here, which is that
when a scenario introduces a new terrain type by merging, this gets
added to the global cache. The alternative is to make a copy of the
global cache, only for the this scenario, as soon as we have to
dirty the cache. However since I didn't see any downside to putting
the merged terrains together and carrying them across scenarios,
I opted not to do that here.