Draft: Added an HSLuv mode to the color selector

This implements two new modes to the color selector: HSLuv and HSLuv Wheel.

HSLuv is basically equivalent to HCL but with a Chroma component scaled between 0 and 100 (as opposed to having a different Chroma for each combinations of Hue and Luminance). The original implementation can be found here (it is also web based, so you could try it out to get an idea of what this would feel like in Inkscape).

This selector is much more intuitive than the more popular HSL, as it groups colors using their real lightness as perceived by the human eye. More details about the advantages of this selector over HSL can be found here. The below image is a screenshot of each selector (HSL in the left, HSLuv in the right), and a grayscaled version below them. And even in the colored version, the right Hue slider feels way more uniform than the left one.

Because RGB is not monotonic in luminance, UIs, statistical graphics and diagrams that use it as a base for the design of their palettes are actually harder to understand, especially for people with color vision deficiency. As HSL and HSV are naïve conversions of RGB, they suffer from the same problems. Using HCL based color selection methods like HSLuv makes the creation of balanced images simpler, with colors that remain consistent in their perceived lightness.

As far as implementations go in the market, the only other drawing program I know of which implemented an HSLuv color picker is Rebelle. Many other applications like GIMP have HCL pickers, but unlike HSLuv, its inconsistent Chroma makes it hard to use for any real work without first investing some time and effort to familiarize oneself with it.

Here are a couple of screenshots to show how this looks like in Inkscape, for both the scales selector and wheel selector: