City::inside_radius() returns true if a point or int pair is inside our radius of control.  Use this in City::display_map().  City::add_area_to_queue() returns false if we can't place for any reason.
City::add_area_to_queue() returns bool.  New function City::inside_radius().
Interface::set_temp_info() replaces the text in text_info until a key is pressed, at which point it's reset to what it was before.  Used when we fail to enqueue an area.  Improve comments in header.