Hi guix!

For folks into music production, this patch set adds drumgizmo, an high
quality MIDI drum.  It comes with a standalone application and an LV2
plugin (VST should also be there, but I could not build it).  I tested
both the cli and the lv2 plugin (inside carla) and they work fine.

Along with it, I added dgedit, a drumkit kit editor.

I also added three drumkits.  Those are big (2 to 5Gb), and the build is
just extract+copy, so I don't know if I should add #:substitutable? #f.
There's no standard path for those, so I'm placing them under
share/drumkits/DRUMKIT_NAME.

guix lint does not complain and they all build fine

Thanks!