Update Interface::display_area_stats() to include information on the tile currently selected; for instance, when placing a farm include a list of crops available and the tile's farmability.  Also, call the function every cycle so that it's updated if we select a new tile.
Make City_map::tiles[][] private, and add a function City_map::get_tile() to access them.  Also, new functions for Map_tile: get_terrain_name(), get_crop_info(), and get_farmability().  Map_tile::get_info() uses the latter two.