Fix bug when a bus with zero channels was added.

Also replace a few Out.kr wit ReplaceOut.kr.  This makes sure that the control
signals are never mixed together (added).
Comment out two server.sync to make sure synth do not accumulate on server.

This works but needs to be tested further.
Call .buildSynthDef from the Mediator instead of Preset.

Move (again) the call of PresetInterpolator.buildSynthDef away from Preset.sc
and into PIMediator.
Fix an incompatibility with the language side interpolator.

While working on PresetInterpolatorServer, I introduced a problem with
PresetInterpolator language side.  This fixes it.
Remove the fork in initWithEvent.

This is to make sure all synths are removed when the synth tree of
InterpolatorServer is built.