TODO

Shuffle operations.  It would be nice to have both a random shuffle
operation and an interleaving enqueue operation.

Sorting operations.

Implement `bongo-file-name-roots' so that forcer can try Bongo. :-)

Customizing `bongo-header-line-mode' should have immediate effect on
existing Bongo playlist buffers.

Implement scrolling for the header line.

Better error messages when players fail.

Generalize intra-playlist queue.

Implement intra-track region repeat.

Provide a way to specify that intermediate headers
(cf. `bongo-insert-intermediate-headers') should be inserted even for
singleton albums.

Implement support for crossfading.

Optimize `bongo-line-get-property' as much as possible, as it is
called lots of times all over the place.