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

Thanks to Themaister for rewind functionality, fetzerch for file length check, and topfs2 for fixing a crash on game client load.
[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 new type of add-on, xbmc.addon.game, plugins and scripts that can be run from MyGames window
[games] Add MyGames window to the GUI
[game clients] Add win32 and linux repositories for game client add-ons
[osx/ios/atv2] - sync xcode projects
[python/games] Expose GameInfoTag properties to Python list items.
Add repository for new ROM Collection Browser with "Game Add-on" support
[python] Allow Python xbmc.Addon objects to be constructed from uninstalled add-ons.

Only getAddonInfo() is available to uninstalled add-on objects. Attempting to get/set settings or location strings will have no effect.
[python] xbmc.Addon.getAddonInfo() can return "extensions" and "platforms" properties for game client add-ons
[JSON-RPC] New parameter for Addons.GetAddons, "uninstalled", to get add-on info from remote repos.

Example: Addon.GetAddons({"method": "Addons.GetAddons", "params": {"type": "xbmc.addon.video", "enabled": "uninstalled"}})