Use music file tags to better display/group played tracks

Rhythmbox is populating my played tracks in Zeitgeist, and while gnome-activity-journal groups them as music files, it displays them as file names. Since track/album/artist metadata would likely be available in the music files themselves, it would be nice if g-a-j used that to display and group audio tracks, when available:

Here's a poor ascii mockup showing 10 played tracks grouped by album, then an individual track that was played, then the first two tracks from another album being played:

{{{
Afternoon
  [v] Listened to music (13)
     [>] Listened to /Keep it like a Secret/ by /Built to Spill/ (10)
         Listened to /Avalanche/ from /Songs of Love and Hate/ by /Leonard Coen/
     [v] Listened to /Thick Mosquito Sky/ by /Crab Smasher/ (2)
           /The Dancing Girl and the Burning Town/
           /Europa/
}}}