Update FFTView.swift backgroundColor to change the view's secondary color

[NickCulbertson]

This change allows backgroundColor to change the entire view's secondary color. Use case would be if you want a white FFTView with only the bars colored, you could init the FFTView like this:

FFTView(conductor.reverb, backgroundColor: Color.white)

(I'm submitting this to the main branch because it's the one that uses backgroundColor from the previous PR)

Thanks, Nick

[aure]

Hmm, this is a bit problematic now because the develop branch had other updates. I'm going to merge everything I can back in to develop and then all work should be done there.

[NickCulbertson]

Sounds good! Let me know if you need anything from me.

Thanks, Nick