jackd and volume control

As far as I understood there are 3 opinions:

1.) Don't change anything
2.) Add some API-calls to control the volume of every port
3.) Add an API-call to control the volume of system:playback_*

I'd say that 2.) will introduce too much complexity. This is what jack
clients should do.

But some time ago I proposed 3.)

- Not every audio adapter has a software controllable hardware mixer,
  e.g. Focusrite Scarlett Solo, 2i2, 2i4. So alsamixer can't do the job.

- No need to break clients' connection to system:playback_* and make
  connections to a volume control application like jack_mixer.
  That's really nasty when the application connects only at playback
  with changing port names, like Audacity. Then you need tools like
  jack-plumbing.

I can live without that but that's what new users would appreciate,
especially when there are some apps for the system tray/dock/whatever.