Wip/aplazas/libretro 1 7 2
This add UNIMPLEMENTED.md, describing parts of the Libretro API which are not yet implemented by retro-gtk. Also update libretro.h to the version from RetroArch 1.7.2.
After this, all updates to libretro.h will require an update of UNIMPLEMENTED.md.
@theawless @exalm, what do you think about this?