v0.28.0: Cassette share selector — choose what friends can cassette

- Sync Settings gets a share selector on each friend page (and on General
  when "Share the same selection with all friends" is on; each friend's
  own selection is kept, greyed, and comes back when it's turned off).
- Library: a genre/artist/album column browser with a checkbox beside
  every artist, album and track, search, Select All / Select None and
  "Share whole library". Playlists: a checklist with "Share all
  playlists". Nothing is shared by default.
- Checking an artist or album is a snapshot of its tracks; the whole
  library and shared playlists are live. A shared playlist's tracks are
  checked and locked, with a tooltip naming the playlist(s).
- cassette/share.py writes the trimmed copy (library.json, playlists.json)
  into each friend's folder: tags yes, listening history and local paths
  no. Written on OK and after library edits (debounced), and only when the
  bytes would change.

lintunes/__init__.py

@@ -1,3 +1,3 @@
 """LinTunes — iTunes-style music library manager and player for Linux."""
 
-__version__ = "0.27.0"
+__version__ = "0.28.0"