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

struct AudioOutput should not know or care about multi-threading.

This allows removing the "base.pause" check from the JACK output
plugin, and will allow removing similar accesses from other plugins.

Prepare to move some configuration settings from AudioOutput to
AudioOutputControl.

The AudioOutput struct (which is exposed to all plugins) should not be
aware that it's being controlled by another thread.

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.

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().