Color picker for the classes

[nicOwlas]

Tasks

• [ ] Add a color field to a LabelClass
• [ ] Decouple class index and color
• [ ] Add to the class creation and edition modal the possibility to select/edit it's color

[bbenoist]

@nicOwlas Here is the code for triggering the system color picker and a live preview: https://codesandbox.io/s/withered-leftpad-91wm9?file=/src/index.tsx

I've also took 5mn to create an example rounded custom color picker component.

⚠️ Please note that once you click one of the buttons, the window/dialog that pops-up is the system one. We can't act on its design.

[bbenoist]

On Windows 11, the color-picker popup looks like that: