Port to soup 3.x

Carlos Garnacho requested to merge carlosg/gnome-music:wip/carlosg/soup3 into master

Port storeart and scrobbler to soup 3.x. Since all Music dependencies seem ported to libsoup 3.x in recent setups (F37, GNOME Nightly), this is necessary to avoid type clashes and other indirect side effects like the application being stalled/frozen (very reproducible locally playing songs from albums missing album art).

Note: The scrobbler changes are untested.

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