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

Thanks to Themaister for rewind functionality.
[retroplayer] Added RetroPlayer, a new player core that enables game clients to play video and audio.

Thanks to elpendor for RGB565 support and ChrisMyhre for catching a compile error.
Add new type of add-on, xbmc.addon.game - plugins and scripts that can be run from MyGames window