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

[Jeremy Bicha]

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 file to our packaging
and I'd like to do that work and the uploads.

I think adding that to each package is the best place for that
metadata. With the exception of libretro-core-info, metadata for a
package belongs in that package not somewhere else.

Please take a look at this soon since I'd like to fix this issue in
time for Buster's release.

https://salsa.debian.org/games-team/mgba/merge_requests/1
https://gitlab.gnome.org/GNOME/gnome-games/issues/79

Thanks,
Jeremy Bicha