Eliminate dependency on io_thread_get().

Remove dependency on ReplayGain global variables.

For easier state management inside filter plugins.

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

There are code paths (mostly error cases) in which it is possible to
initialize an AudioOutput and then kill it without ever calling
audio_output_new().  In such a case, its destructor will attempt to
free a mixer that was never initialized, leading to an attempt to
take out a lock on a mutex that was similarly never initialized,
which hangs forever.

Fix by always initializing the mixer appropriately.

Use a listener interface instead of GlobalEvents.

Passing a void pointer is unsafe.

Make it "const".