Clean-up and polish of game_config_manager class.

Add extra braces.
Substitute for loop with BOOST_FOREACH.
Update CMakeLists.txt and VC9/wesnoth.vcproj.
Split some long lines.
Change pass by value to pass by reference.
Fix formatting of some comments.
Change some for loops to BOOST_FOREACH in game_controller class.
Introduced enums for load_game_cfg().
Fix wrong/misplaced function calls. Rename a method.