HSL color wheel implementation.

This is a standard Hue - Saturation - Lightness model
(see for instance entry on wikipedia here: https://en.wikipedia.org/wiki/HSL_and_HSV)
Note though the difference between HSV and HSL saturation, which are not the same.

The advantage of having this color selection scheme is that artists can select
shades and tints of a color easily by using the lightness slider. Also colors
are arranged on (approximated) perceived lightness on the color wheel.

Beware, Old files opened with this preference saved will crash!