[Feature request] Add option to save files as grayscale

[zaps166]

PNG, TIFF, JPG formats support grayscale natively. Using grayscale when saving reduces file size and speeds-up the saving process. Currently RawTherapee always saves all RGB channels in PNG/JPG/TIFF files where all channels are the same (by using Black-and-White Abstract Profile or mixing channels to same values of all channels or using Black-and-White tool).

In GIMP, you can achieve it by doing Image->Mode->Grayscale, and export the image.