Provide AppStream validation

[cassidyjames]

Much like the lint action, it would be handy to provide an AppStream data validation action so we can ensure things are valid before they're merged. Would avoid issues with malformed AppData, missing tags, etc.

[kgrubb]

Should this be its own action even though its super tiny/short? I'm thinking yes, but I'm not certain 😄

[decathorpe]

It would be great if appdata could be validated both with appstream and appstream-glib - even in the "relaxed" mode, the latter is stricter for some checks (and for some other checks, it's the other way round). Validating with both major implementations should cover most issues.

(appstream is used by AppCenter and KDE, appstream-glib is used by GNOME / PackageKit - so basically everybody except Appcenter and KDE)

[kgrubb]

I added appstream-util validate-relax --nonet, which should cover appstream-glib I think? Where's the best place to take a look at the other validator?

[decathorpe]

@kgrubb I've left some feedback on your PR, including how to invoke the non-glib appstream validator CLI :)