flatpak: Add flatpak-builder manifest

Add the flatpak-builder manifest for Games as well as the needed
Libretro core descriptors and custom makefiles.

This allows to remove these same files from the gnome-apps-nightly repo,
see https://wiki.gnome.org/Initiatives/GnomeGoals/FlatpakManifests for
more details about this initiative.

https://bugzilla.gnome.org/show_bug.cgi?id=779403