Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

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

Will move attributes from struct AudioOutput that are specific to the
OutputThread.  The new struct AudioOutputControl is a holder for the
AudioOutput pointer.

This prepares for making the output list more dynamic, to allow moving
outputs to between partitions.

Reduce overhead by eliminating MultipleOutputs::ResetReopen().

Eliminate a dependency from MultipleOutputs on PlayerControl.

Remove dependency on ReplayGain global variables.

Use a listener interface instead of GlobalEvents.

Another big chunk of code for multi-player support.