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.

Wake up the PlayerThread only if it is really waiting for the decoder.
This greatly reduces the number of system calls in the DecoderThread.

Share the Mutex between the DecoderThread and the PlayerThread.  This
simplifies synchronization between the two threads and fixes a freeze
problem: while the PlayerThread waits for the DeocderThread, it cannot
answer requests from the main thread, and the main thread will block
until the DecoderThread finishes.

Replaces GLib's GError.

Don't let the "client" pass its own GCond.  This was not used
consistently.

Make sure the decoder "owns" the song object, so nobody else can free
it.

Replaces _current_song().

Don't read song.start_ms and song.end_ms, let the player thread manage
this logic instead.