Obsolete class CloseSongEnumerator, which was a kludge.

Don't use GLib.

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 pass a GError** to g_key_file_get_X().  We don't need to dispose
something we didn't request in the first place.

Remove the forward_list::reverse() call.  It was not necessary,
because pls_parser() already reads the playlist in reverse order.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces struct playlist_provider.

Replaces GLib's GError.