Implement lowpass filter

[hvalfangst]

One can utilize the existing idle knobs for controlling cutoff and resonance

[hvalfangst]

LPF with cutoff knob has been implemented in ce09fb2

[hvalfangst]

Implemented the ability to toggle the LPF on/off in fc0a6de. A bulb will light up on its activation. Deactivating the filter will reset its associated cutoff coefficient value in order to also reset the knob sprite

[hvalfangst]

Will be improved in the current state