Experimental `fftns()` version

Adds the fftns() function that's normalized and smoothed peak normalized with peak smoothing (with the default options in the code) per https://github.com/TheNuSan/Bonzomatic/blob/29f3e46f6fbbf3fc3aec46af2965f9ed0dfc5beb/src/platform_common/FFT.cpp#L137.

You'll need to start it with --fft for FFT to be enabled.

edit: does NOT yet do actual smoothing.