Reorder Color -> Monochrome menu
New Monochrome -> Grayscale tool

Same idea as the java equivalent (SubsampleBinaryToGray).  Since
PhotoDemon provides a mechanism for converting images to monochrome, it
should also provide a mechanism for getting images back from monochrome.
Use better default values for the Unsharp Mask tool
Fix possible OOB error by monochrome threshold auto-estimator