Double frequency shifter module.

Sliders under the graph:
-Frequency Shift 1: First frequency shift value.
-Frequency Shift 2: Second frequency shift value.
-Filter Freq: Cut-off frequency of the lowpass filter.
-Filter Q: Q factor of the lowpass filter.
-Feedback Delay: Delay time (in seconds) causing the feedback.
-Feedback: **à vérifier**
-Feedback Gain: **à vérifier**
-Dry/Wet: Mix between the original signal and the shifted signals.

Dropdown menus and toggles:
-Balance: Adjust the signal amplitude by comparing it with a sinusoidal wave with a fixed amplitude (see "compress") or with the amplitude of the source (see "source").
-# of Polyphony: Number of voices played simultaneously (polyphony); only available at initialization time.
-Polyphony Spread: Pitch variation between polyphony voices (chorus); only available at initialization time.

Graph only parameters:
-Overall Amplitude: The amplitude curve applied on the total duration of the performance.
