Bug#911329: add .libretro metadata for better integration with gnome-games

[Jeremy Bicha]

Hi Sérgio,

It's been a while since we last discussed GNOME Games integration with
libretro cores.

I would like to add the Appstream metadata and .libretro files to 9
libretro cores in Debian. 4 of these have already been done in Ubuntu
and would have been done in Debian too except you objected previously.
Would you be willing to reconsider? Currently, gnome-games-app is a
lot more useful on Ubuntu than on Debian because of this issue.

Also, I think it would be useful if we had a git repository for the
Debian packaging for these libretro cores.

Thanks,
Jeremy Bicha