[VS] add add-on includes to Visual Studio project
XBMC Game binary add-ons
Libretro support for game add-ons
GameClients work-in-progress

* Add Game API document in GameClient.h
Game binary add-ons: removed support for loading files from memory (squashme)
libretro: removed support for loading files from memory (squashme)
GameClient: work-in-progress (squashme)
RetroPlayer work-in-progress
Rename Game API function AllowVFS() to SupportsVFS(), and remove unused RequireArchive()
[library.xbmc.libretro] always call Get*() functions
Make game callbacks returning strings return char* that need to be freed with FreeString()

Also fix pixel_format callback returning void instead of bool
Some work in GameClient.h
[cosmetic] Clean up game info tags. Also fixes bug in CGameInfoTagLoader::GetPlatformInfoByExtension()
General game clients cleanup
Remove EnvironmentSetPerformanceLevel() Game API callback (replaced RETRO_ENVIRONMENT_SET_PERFORMANCE_LEVEL)
Make EnvironmentSetSupportNoGame() a Game API call instead of a callback

Also fixes typo with EnvironmentSetPixelFormat()
Return strings of static duration
"Finished" cleaning up GameClient
Removed GameFileLoader.cpp and merged functionality into GameClient.cpp
[cosmetic] Game settings