new Oklab gradient picker with sliders for L, a, b

[KrabCode]

overview
relevant java library

It is probably worthwhile to make it a radio-based option to switch between color spaces at runtime, even if that destroys the color or has to find an imperfect approximation in the new color space.

[KrabCode]

perhaps we can only poach some useful parts of the library's shaders and in this way avoid including a whole new dependency to babysit

upon switching from the default RGB we could hide the RGB sliders and show new ones to control the L, a, b, but that would require a major rework of the gradient java class as well as the shader that draws the gradient