[FR] Add lower & upper exposure cutoff to GUI

[da-phil]

According to the manual:

This option can be helpful if the user wants to exclude underexposed or overexposed pixels from the fusing process in all of the input images. The values of LOWER-CUTOFF and UPPER-CUTOFF as well as the gray-scale projector determine which pixels are considered “underexposed” or “overexposed”. As any change of the exposure-weight curve this option changes the brightness of the resulting image: increasing LOWER-CUTOFF lightens the final image and lowering UPPER-CUTOFF darkens it.

Defaults: ⟨0%⟩ for LOWER-CUTOFF and ⟨100%⟩ for UPPER-CUTOFF, that is, all pixels’ values are weighted according to the “uncut” exposure-weight curve.

Some useful examples: