WIP: Use appstream metainfo to filter applications too

TODO:

• Don't filter before metainfo cache is up to date
• keep cache up to date on app add/removal (ideally fixed in appstream: see https://github.com/ximion/appstream/issues/337)
• make sure flatpak user metainfo gets added (see https://github.com/flatpak/flatpak/pull/4350)
• cache successful lookups to avoid rechecking the data (this is pretty fast but would make sense if we cache the lookups for desktop file information as well)

Closes: #611 (closed)

Signed-off-by: Guido Günther guido.gunther@puri.sm