Fix indexing and thumbnailing of the multimedia demopack for Frampton

The Mildenhall Frampton application relies on Tracker to index the media files that the demopack installs in /home/shared for its songs list and it is currently broken, see APERTIS-5826.

To fix that:

• install a GSettings override to point Tracker to /home/shared
• install a minimal service in the user session to monitor indexed files in Tracker and request thumbnails for them

See also OBS-10331 for the fix to actually make tumbler extract the cover art from audio files again.