This adds a secondary mechanism to check whether MPD's DB update job has concluded. The first one doesn't fire if pointed at an empty folder (empty DB -> empty DB, nothing changes, no idle message).

It also adds a live(ish) discovered song count to that page, updated every 5s to avoid overloading MPD during DB update, so the page feels less frustrating to look at & can be debugged more easily.

Other changes

• Experimental new sidebar, following libadwaita's newly standardised widget.
• TheAudioDB provider (far better artist avatar hit rate!). Disabled by default. Enable with your API key then re-fetch metadata to get them.
• A few other stuff pending a v1.0 release, to be announced later.