Add replace versions of all FX synths and new basic_mixer:

* In anticipation of moving to a single pair of audio busses per job
  along which all FX are chained (rather than a pair of audio busses
  per FX synth)
* Also in anticipation of each job having a global mixer synth to 
  control amplitude of all synths created by the job. This will allow
  nice fading out on job kill and eventually job level audio mixing.