Improve color picker UI

[jacomyal]

• Dragging the color in the color space shouldn't select anything in the DOM
• On a gradient or a ranking, it is possible to have multiple color pickers opened at once
• Before being clicked, color pickers look too much like radio buttons (some people missed for instance the clickability of the target color of shading, which is white by default)

[sim51]

Only the last point is remaining, but we already worked a little on it by making a square with a black border.
So even if it looks like a checkbox it's better than before.
Moving this ticket in the "To validate" column