added frequency sliders for the tone stack

[fichl]

sliders to control the frequency of the individual tone stack knobs.
show / hide with the new icon next to the EQ-text
double click on sliders restore original default values

[sdatkinson]

Thanks for the PR.

I don't think I'm going to merge this because I don't like the assumption that there's "a" frequency associated with each knob. For example, currently, the mids' Q adapts based on whether the knob is cutting or boosting, and general "tone stack" EQs can be modeled by altering the structure of the EQ filters as the knobs move (see e.g. TSC).

So by introducing more controls, that makes these assumptions about how it operates become constraints, and I don't want to box in this plugin like that--better for others to extend it and make those choices, while this gives them the option to do so.

FWIW, it'd be neat to refactor the tone stack code to be a little tidier in ProcessBlock--perhaps that would make it easier to be able to roll this customization on top of this repo. I've sketched an Issue for this: #441

Then you'd be able to subclass the tone stack as you'd like, set up the parameter stuff accordingly, and that should make it relatively easy to maintain this customization.