Add Freqpalette effect

[megabug]

This sound-reactive effect uses the average value from user-selected FFT buckets to select a single color from a palette.

I couldn't find any existing SR effect that could do this - the volume-reactive ones only consider the entire signal and not a specific frequency range.

I use this on a set of shelves, where each shelf has a different FFT range that it lights up in response to. This creates a cool effect where each responds to different parts of incoming audio i.e., kick, snares, synths etc.

Bad video demo: (warning, flashing lights)

https://user-images.githubusercontent.com/15011974/229482894-710e4f1e-8718-42ce-b299-66e6bfdeef38.mp4