SkUGens: add SkUGens (FM7 only)

 - original repo is at https://github.com/kaoskorobase/skUG
SkUGens: adapt FM7 to new plugin interface
SkUGens: FM7 - port help file to SCDoc format
Merge pull request #1 from gusano/topic/skugens

add FM7 (from SkUGens) to sc3-plugins