New function, City::get_minerals_used().  Returns an empty map for now.
Rename City::get_mine_production() to City::get_amount_mined().
Rearrange things, remove near-useless list of possible minerals, add list of minerals stored.
Confirm when player tries to quit.  Interface::minister_mining() has slight refactoring, and now displays a list of minerals mined, used and stored.
New function, Map_tile::get_glyph().
New function, Map_tile::get_glyph().  City_map::get_glyph() uses this function now.