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

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".