Add support for highway tiles in caves

[Adambean]

Would there be scope for adding support for highway tiles (such as cobblestone) in cave views?

These currently appear in the same grey as rock tiles, so underground highways do not show up. The amount of colours for each tile would just get stupidly complicated so it'd likely be better to have a default dark grey or black (though different to slate) for all tiles that don't fall within the typical cave class.