.flatpakrepo support

[davidmhewitt]

Fixes #72

Still do to:

• [ ] Sort out POTFILES after renames and new files
• [ ] Detect if a remote is already added and do something?
• [ ] Show a success view on success
• [ ] Show an error view on error (both error parsing file and adding repo)

It seems like there's already quite a bit of duplicated UI code here, so I don't know if we want to do something more clever. But the success view and error view classes are pretty short and can probably be tweaked slightly to be the success view and error view for both Repos and Refs.

[cassidyjames]

@davidmhewitt I'm a fan of this direction so far. :) I think the error view can probably be shared if that makes sense, but the success view is going to have a different title, body, and actions, so it might make sense to keep it separate. Something like:

Added “Flathub” to software sources

Non-curated apps and updates from “Flathub” will now appear in AppCenter.

Close Open AppCenter

[hanaral]

I'm not sure if this is still being considered, but as it directly effects its app selection this should be an AppCenter dialog instead imo Maybe there should be even more distinction between AppCenter and non-AppCenter apps?

[cassidyjames]

@hanaral Sideload handles sideloading things from outside of AppCenter, including Flatpak refs, bundles, and (with this PR) repo files. This is still definitely wanted, but it'll need to be rebased on master as there have been a lot of changes there.