Port to libappstream

I've been told that the reference AppStream implementation libappstream and the one use by Software appstream-glib are fairly similar, one of the main differences being that appstream-glib adds kudos to applications.

IIUC we want to redesign the kudos so they can be completely computable on the store's side and not something self-awarded by the applications.

It also seems appstream-glib has some inconsistencies with the stanrdard, which seems to be the cause of this weird app-duplication bug: https://source.puri.sm/Librem5/pureos-store/-/issues/8, porting to the reference implementation would likely solve it.

I've also been told @hughsie would not be against such a change if someone else was implementing it, can you confirm‽ :)