Suggestion: More advanced color picker

[novhack]

For now the current color picker is enough but for the future adding more pixel art related features would be nice.

• Hue shifting and shading sliders.
• Color previews in RGB/HSL sliders.
• Preview of original color for reference.
• Different color textures than a simple shade - RGB spectrum and normal map wheel.

[novhack]

A new implementation could remove raw mode and solve #349. Raw mode makes sense for HDR stuff but I don't think something like that would make sense in a pixel art editor.

[novhack]

#184 suggests to have the picker embedded into a panel like in Graphics Gale. I think that this request could be handled by optionally setting the color picker popup to exclusive mode so it's not automatically closed.