system: add SynthProxy as copy of NodeProxy2
system: SynthProxy adaptation of NodeProxy2 code
system: NodeProxy2, SynthProxy: add simple volume control
system: NodeProxy2, SynthProxy: keep volume variable unwarped
system: PatchEnvironment: support environment parenting
system: NodeProxy2, SynthProxy: implement setting/getting controls

When getting controls, either the last set value, or the SynthDef's
default value will be returned.
system: prototype ProxyGui
preprocessing: add function to stop preprocessing
separate sound definitions into sound.scd, streamline main.scd, add GUI