Use some proper terrain type names:
- "grassland" becomes "flat"
- "tundra" (which was displayed as "snow") becomes "frozen" (not sure
it's the optimal name here - can be changed if we find a bettter
name)
- "canyon" (which was displayed as "chasm") becomes "unwalkable"
- "cavewall" becomes "impassable"
- The renamings of the wml key used in the stats will probably stay.
- The 4 added "virtual" terrains will be axed by the next step, once
we get proper terrain type support.
- The labels might be changed (Noy had some ideas about it, it will be
easier when we have terrain types)