Interface for Interface::minister_mining().
Building::get_empty_shafts() reports shafts not used (if any).
New City functions: get_shafts_worked() and get_free_shafts().  Crude version of mineral discovery in mines.  Set up mines properly in City::add_open_area().
Add a border around the help text.
Interface::minister_mining() and Interface::list_mine_minerals().  minister_mining() is very similar to minister_food() and is nearly complete; a few fields aren't populated yet.  Added "Mining" to Minsters menu.  Fixed corruption amount in Interface::minister_finance().  Removed some extraneous lines.  Greatly improved comments in Interface::minister_food().
Map_tile::get_mineral_amount().  Removed some old commented-out lines.
Add value and hidden properties to Mineral_datum.  New constant HIDDEN_RESOURCE is used to indicate a resource (read: mineral) that a mine hasn't discovered yet.
Put in value and hidden properties for minerals.