ImGui color picker based on OKHSL instead of HSL

[JulesFouchy]

Okhsl is a perceptually uniform version of HSL. You can read this great article about it.
This is what Godot 4 now uses by default :

We want to implement the same thing in Dear ImGui to replace their current color picker widget. Their color picker code is a bit messy, so this might mean rewriting a color picker from scratch.