EQ Addition: Lowpass + Highpass

[torridgristle]

Currently the EQ only boosts or attenuates specific frequency bands. An actual lowpass and highpass filter to switch on or off with adjustable order / tightness and frequency would be nice for taming bass in a way that rolls off lower and lower frequencies by more and more decibels with a highpass, and would help with getting a duller / warmer sound by softening the high-end with a lowpass. Bessel filtering though, not Butterworth. An explanation on this is in another issue post regarding swapping Butterworth for Bessel but tl;dr it's smoother and safer.