Handle .flatpakrepo files

Flatpak supports .flatpakrepo files to add a repo without installing an app. GNOME Software seems to handle this and adds the repo. If we want feature parity there, we could support opening and adding a repo file. This would require a new screen specifically for repos that details the implications.

This seems to be a sticking point for them to let us update the instructions at Flatpak.org without directing users to the Terminal: https://github.com/flatpak/flatpak.github.io/pull/355