Spectral filter module based on fast-Fourier-transform (FFT) operations.

Sliders under the graph:
-Filter interpolation: Morphing index between the two filters.
-Dry/Wet: Mix between the original signal and the delayed signals.

Dropdown menus and toggles:
-Filter Range: Limits of the filter (up to Nyquist, up to Nyquist/2, up to Nyquist/4 or up to Nyquist/8). 
-FFT Size: Window size of the FFT (in samples - 16, 32, 64, 128, 256, 512, 1024, 2048, 4096 or 8192).
-FFT Envelope: Envelope type for the FFT (rectangular, Hanning, Hamming, Bartlett, Blackmann 3, 4 or 7, Tuckey or sine).
-FFT Overlaps: Number of FFT overlaps (1, 2, 4, 8 or 16).
-# 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:
-Spectral Filter 1: Shape of the first filter.
-Spectral Filter 2: Shape of the second filter.
-Overall Amplitude: The amplitude curve applied on the total duration of the performance.
