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

Obsoletes the same variable from AudioConfig.cxx.

Remove dependency on ReplayGain global variables.

The old struct config_param remains only for top-level string options.

Since version 2.32, g_thread_init() is deprecated and a no-op.  Let's
upgrade that (optional) dependency so we can remove all those
g_thread_init() calls.

Auto-stop the IO thread in all error handlers.

Use a listener interface instead of GlobalEvents.

Another big chunk of code for multi-player support.

Avoid GLib.