Fixed broken/incorrect orb example image paths
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.

Edit: There were minor issues in campaign_server, now resolved
after rebasing this.
Rename filesystem::file_name to filesystem::base_name

Amended: src/editor/map/map_context.cpp
   src/gui/dialogs/editor/custom_tod.cpp
   src/gui/dialogs/screenshot_notification.cpp
   src/gui/dialogs/wml_error.cpp
   src/serialization/preprocessor.cpp
Remove some unneeded includes
Make filesystem.hpp not depend on config.hpp being included first

Conflicts:
	src/filesystem.hpp
Rename file_create_time to file_modified_time, as that's what it does
Remove a comment that is no longer true
Don't try to iterate over things that are not directories
Split the parts of filesystem.cpp that don't directly interface with the filesystem off into filesystem_common.cpp
add filesystem_common.cpp to cmake
HttT S16: Hide Li'sar on prestart, not start

Otherwise she is visible for a split second at the start of the
scenario.
Fixed bug #22799 and improved the related code
Fixed a comment and commented out a debug message
Fixed the traveler's ring part of bug #22790
Purge stale R_N contents
updated Italian translation
UtBS: Use my TC-aware Skeleton Rider baseframe already used in Liberty

Death to the Wesnoth 1.0 skulls.
UtBS: Use UNREACHABLE instead of 100 MP cost in desert_float movetype

Fixes the following warning for new units with the movetype:

> warning config: Terrain 'Wdt' has evaluated to 100 (cost), which is more than 99; resetting to 99.
UtBS S3: Round lower random range bound

Fixes an endless stream of warnings during gameplay on medium
difficulty:

> error engine: Malformed range: rand = "17.5..35"
LoW 10: Corrected a wrong spelled unit id in a recruit list.