[3.38] grltrackerwrapper: Fix artist queries to retrieve art
requested to merge wip/mschraal/grltrackerwrapper-fix-artists-query-for-art-retrieval-3-38 into gnome-3-38
Artwork for artists was not retrieved anymore, this was due to the artist field of the Grl.Media no longer be filled. This can be traced back to bf973c12, the initial porting of the Tracker artist queries to Tracker 3.
In Music a distinction is not made between artist and albumartist tags. The latter is always preferred, but might not be available. So, the artist tag should always be considered filled. In this case however the query was altered to only provide albumartist to the Grl.Media.
The art lookup resolve operations require the artist field of the Grl.Media to be present, so they would return empty as the Grilo plugins do not support albumartist.
Restore the previous behaviour of the queries.