Reduce bloat.

Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Don't use IgnoreError() when there's an overload that does not try to
give us one.

Decouple some more from GLib.

Allows forward-declaration.

Replaces struct playlist_provider.

Replaces GLib's GError.

Use the C++ API.