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

Thanks to Themaister for rewind functionality and fetzerch for file length check.
[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.
[retroplayer] Added keyboard and joystick input handling to RetroPlayer built on the new joystick abstraction layer.
[games] Add MyGames window to the GUI