Move everything in src/filesystem.hpp into its own namespace

Conflicts:
	src/addon/manager.cpp
	src/campaign_server/campaign_server.cpp
	src/editor/map/context_manager.cpp
	src/editor/map/map_context.cpp
	src/font.cpp
	src/game.cpp
	src/game_config_manager.cpp
	src/gui/dialogs/editor/custom_tod.cpp
	src/gui/dialogs/lobby/lobby_data.cpp
	src/gui/dialogs/mp_create_game.cpp
	src/gui/widgets/settings.cpp
	src/hotkeys.cpp
	src/multiplayer_create_engine.cpp
	src/multiplayer_lobby.cpp
	src/network.cpp
	src/playcampaign.cpp
	src/savegame.cpp
	src/serialization/preprocessor.cpp

I produce this by cherry-picking gfgtdf's commit of AI's original
commit, from master to 1.12 as usual. git figured out about 2/3
of the file correctly, the rest I mostly had to do by hand. I have
now inspected each line of the diff, it looks good to me.