v0.32.0: folder tree in Rabbit Sync Settings, quieter art pass, tink, VLC-style Preferences

- Rabbit Sync Settings shows the sidebar's folder tree. A folder has its own
  tristate checkbox; a fully ticked folder is stored as device_sync.folder_ids
  and follows the folder, so playlists added to it later sync too.
- The album-art pass compares covers against the sizes the planner's device
  scan already read (plan.art_sizes) instead of an MTP stat per album, caches
  "no art" as a .none marker, and only unlinks covers that are really there.
  The summary reports covers actually updated rather than every cover.
- A quiet tink when a Rabbit sync, export or URL import finishes, with an
  on/off switch (sound_effects, on by default).
- Preferences is now General / Appearance / Integrations, a list on the left
  and the page on the right, opened big enough for the tallest page.

lintunes/__init__.py

@@ -1,3 +1,3 @@
 """LinTunes — iTunes-style music library manager and player for Linux."""
 
-__version__ = "0.31.9"
+__version__ = "0.32.0"