Add HSV Path to color gradients

[jsbueno]

Terminedia color gradients are pretty much stand-alone "abstract" usable gradients - they can be enriched to be able to better describe gradients.

A low hanging fruit is to have HSV color-model gradients, instead of just RGB color transitions.

GIMP has those - as well as other features that can be incorporated, such as using different color transition curves per segment, instead of just linear color transitions.