Add a world location to City.
Add two new menus - World and Help.  Both have some options, neither actually does anything.
In Kingdom::add_city(), assign a location to the city.
Assign a location to the player's city.
World_map::draw() no longer takes a window as a parameter - it always owns its windows.  Instead, it takes an optional Point to start at.  If no Point is given (or if the Point given is out of bounds), it uses the odl behavior of starting at continents[0].