This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Previously, both methods accessed a lot of attributes which require
mutex protection.

Obsoletes the same variable from AudioConfig.cxx.

Prepare full C++ exception support in the decoder thread.

The state ERROR can only be entered during decoder initialization, and
Seek() may only be called after initialization has been finished.

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.

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

Use LockSynchronousCommand() instead of SynchronousCommandLocked().
Fixes regression from commit ef663810 (dead lock due to cond_wait with
unlocked mutex).