add game_board struct to hold map, units, teams

For now this will just help us to put code in logical places and make
the save and load process simpler. This change is invisible outside
of play_controller.

Please don't backport this to 1.12.
cleanup for loops made ugly in previous commit

This is a strict refactor.
move some functionality out of play controller to game_board

This moves some simple code which manipulates the end turn status
of the units on the map.
move some of the savegame code from play_controller to game_board
replay controller uses game_board to do resets

This is not a strict refactor -- the game map will also be reset
to the starting configuration now, when we reset the replay. It
doesn't look like this was the case in the old version, so it's
possible that if a scenario used [replace_map] or similar, the
replay could go out of sync after hitting the reset button.

Regardless of whether the old version was buggy, the new version
should be just as good if not better, and was tested.
move game_board to its own file, update project files
move code managing preference encounters out of play controller
add access controls to game_board
give global fcn's (find/get)_visible_unit a home in game_board

Unit tested and playtested after moving the functions decalarations,
and definitions, adding links to play_controller::game_board from
resources.?pp, and executing find and replace commands:

git grep -lz 'find_visible_unit(' | xargs -0 perl -i'' -pE "s/find_visible_unit\(/resources::gameboard->find_visible_unit\(/g"
git grep -lz 'get_visible_unit(' | xargs -0 perl -i'' -pE "s/get_visible_unit\(/resources::gameboard->get_visible_unit\(/g"
integrate game_board into mouse_handler
Merge pull request #180 from cbeck88/add_game_board_struct_alternate

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.

This refactor is ongoing WIP. The goals are

(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.
(3) Refactoring clarifies what the existing code is doing, therefore it may help us to find bugs in the current system, which may be fixed independently of the refactor in 1.12.
move carryover into its own file
remove unused variable
remove unused variable continuance
remove resources::state_of_game

we don't want to expose the fileds "snapshot", "carryover_sides".
instead we only add classification, and mp_game_setting to resources
which are members of game_state.

in order to do that, we have to add a do_autosave method to
play_controller.

this commit also moves some code from game_state::write_snapshot to
play_controller::to_config which was intended to be in a different
commit.
move next_scenario_append/setting to carryover