Allow fixing the current note & add MUSIC spectral view
- Add a "Fix" button which forces FMIT to only search for frequencies near the selected frequency. Add an up and down arrow to modify the selected frequency (see bottom right in the screenshot)
- Add another spectral view which shows the output spectrum after running the MUSIC frequency estimation algorithm (after low-pass-filtering and downsampling)
I hope this is useful. At least I found that for very low or high notes FMIT often fails to predict the correct tone and then it is helpful to be able to force FMIT to use a specific target tone. The MUSIC spectrum is an additional hint for unstable tones. Both was evaluated when tuning my piano XD
Just tell me if I should modify something on my pull request.

One main issue with this PR: It is actually 2 different features mixed in a single PR.
be able to force FMIT to use a specific target tone
I don't think I see the use case here, how/why would you need this feature that is not already closely covered by the other features? Do you mind elaborating an example?
What is the advantage of "MUSIC Spectrum" ? Namely, why not using the "Fourier Transform" view?