Bug#911327: add .libretro metadata for better integration with gnome-games

Sergio, I opened a merge request for this issue, because I know when we discussed this in 2017, you didn't think it was appropriate to add these .libretro files to each package. Since then, we're already adding AppStream metadata to these packages so I don't see the harm of adding one more small

Bug#911327: add .libretro metadata for better integration with gnome-games

Package: libretro-mgba Version: 0.6.3+dfsg1-2 Please consider adding .libretro metadata for better integration with gnome-games. See https://gitlab.gnome.org/GNOME/gnome-games/blob/master/flatpak/libretro-cores/mgba.libretro: [Libretro] Type=Emulator Version=1.0 Name=mGBA