libretro: added bsnes, fba, gambatte, genesis-plus-gx, prboom
XBMC AdvancedLauncher RetroArch launchers.

These are wrappers for retroarch wrappers, they are needed since launching a
process from XBMC(as a display manager and probably otherwise), using
AdvancedLauncher, results in it and it's parent recieving the same gamepad
input.

These wrappers will produce no sound on XBMC and AdvancedLauncher setups not
using a sound daemon, since XBMC gets paused while still holding onto the sound
device.
xbmc: enable plugins to be picked up from the store.