Use appstream.dart for packagekit icons and screenshots

[Feichtmeier]

Unlike snapd, packagekit does not provide any icons or screenshots of the apps. So to get those we need appstream.dart, which should be used in the package_service.dart here to fetch the metadata for X packages ad store them

IMO since snapd is so very fast with getting icons and screenshots I would not slow this app down by using appstream for snaps - Opinons ? @oSoMoN @jpnurmi @Jupi007

Depends on https://github.com/canonical/appstream.dart/issues/15

[Jupi007]

Maybe a stupid question, but can't you just conditionally fetch the app icon: false for snap, true for other?

[oSoMoN]

I haven't looked in the implementation details, but from your description of the problem I would indeed recommend using appstream only for packages where the icons/screenshots are not available otherwise (i.e. not snaps).