If I search for "midnite", all artists except Beck should be hidden from the
Artists listbox, and all albums except Midnite Vultures should be hidden from
the Albums listbox, since no other songs match except those from that album.

If I search for "Aero", the only two artists should be Aerosmith and Neutral
Milk Hotel; the only albums should be (in my case) Unknown and The Aeroplane
Over The Sea. (The only songs that match are from this album or an unknown
album, and all are from Aerosmith or Neutral Milk Hotel.)