Color Spaces for mixing colors

[DevCharly]

This PR adds support for Oklab and linear sRGB color spaces to color functions mix(), tint() and shade(). (issue #1109)
It also adds preview for color functions to FlatLaf Theme Editor.

Color Spaces

To use Oklab or linear sRGB color spaces in color functions mix(), tint() and shade(),
pass oklab or lrgb as last parameter:

mix(#f00,#0f0,40%,lrgb)
mix(#f00,#0f0,40%,oklab)

tint(#f00,40%,lrgb)
tint(#f00,40%,oklab)

shade(#f00,20%,lrgb)
shade(#f00,20%,oklab)

Following screenshot shows some differences between the color spaces:

Color Function Preview in FlatLaf Theme Editor

If the editor cursor is at a color function, a preview of the result and the whole percentage range is shown at the bottom of the "Preview" area. This makes it easier to choose a good percentage value and color space.

The color function is shown at top.
Then the initial color and the resulting color (with percentage).
The bottom gradient shows the color function result in range from 0% to 100%.