OkHsv, OkHsl (rebased)

[weltkante]

WIP rebased version of PR #43 to implement issue #62

The graytone conversion for disabled controls has been reimplemented on top of the new slider logic, since that conflicted during rebase. Some of the original PR's refactoring has been undone to allow clean rebase. Check https://github.com/PixiEditor/ColorPicker/compare/ok-color-spaces...weltkante:ColorPicker:ok-color-spaces-1?expand=1 for what has been reverted from the original PR, if needed.

unfinished parts:

• Updating the Avalonia side of the implementation is still to be implemented.
• Running the demo app seems to have some issues to resolve
  • changing value on okhsv view while saturation is zero causes the hue slider to jump around randomly in hsv/hsl mode
  • visual artifacts in the 2d-gradient image (a green bar on the right side) when the sliders are in okhsv view but the 2d-gradient is in normal hsv/hsl view
  • okhsl and okhsv seem to use the same 2d-gradient image, double check against against reference (linked in issue) to see if thats the correct approach. Since classic hsl and hsv have different 2d-gradients this is likely to be a mistake.
  • not all dropdowns in the demo app are offering okhsv/okhsl
  • unrelated to this PR, but there seems to be no way to test disabled controls