nb_mxsynths

mx synths as an nb player

this is more or less port of infinitedigits' mx.synths (thanks zack!) with the following changes:

added: - save and load presets - dynamic parameter menu (synth specific parameters are displayed) - modulation via nb:modulate(val) -> pairs well with sidvagn. I surfaced the mod amt parameter, so it's midi mappable, which means modulation can be used with scripts that don't use nb:modulate. The main use case is to temporarily morph between different parameter settings. - fx via fx mod - 6 voice limit

removed: - polyperc (already exists as separate nb voice) - internal fx bus