[game clients] Added new Game Client add-ons based on the libretro API.

Thanks to Themaister for rewind functionality, fetzerch for file length check, topfs2 for fixing a crash when loading game clients, and eibma for fixing linux compilation errors.
Squashme: Add strings
[retroplayer] Added RetroPlayer, a new player core that enables game clients to play video and audio.

Thanks to elpendor for RGB565 support, poisson for RAII improvements and ChrisMyhre for catching a compile error.