Relative color remap

This plug-in changes colors in an image based on the HSV difference between two colors. This is particularly useful for applying color themes to existing images.
It avoids burning out high saturation and value at the expense of compression them somewhat when the difference goes towards higher saturation and value.
Current bugs/missing features include:
Undo does not work.
It can't be used on indexed images, even though it would frequently have enough colors.
It's slooow. Since it looks at only one pixel at a time, it should use some "pixel-iterator" that goes through the tiles in an optimal manner.
Selection only works in binary.
Grayscale support is untested and probably broken.
There is no preview.
Prevention of highlight burnout should be optional.