Add game_board class

This is a refactor to introduce an object encapsulating the unit map, the game map, and the list of teams. Introducing this object permits us to move a substantial amount of code out of the play_controller object, and also to give a home to some helper functions in unit.?pp that previously sat in the global namespace. It also allows us to simplify the construction of some of the clients of play_controller.

The main goal is to (1) better organize the architecture of the engine, to make saving and reloading easier.
(2) Facilitate the introduction of an improved pathfinding mechanism, which will need to sit between most of the other engine modules and the unit map / game map.