Add a dropdown to change the ring color

Work on #10 (closed)

I did not put a tooltip for now but a simple toggle group that appears when toggling the rings dropdown.

Things to discuss:

• Should the dropdown disappear when the rings are toggled off ?
• What do you think about the Primary/Colormap groups ? I thought we needed a way to differentiate the colormap colors. We can indeed have two white (or black) squares depending on the colormap (ex. with Blues) but one will depend on the colormap while the other will not.
• I tried to use a computed store property as suggested in !45 (comment 143091) to compute the color string but naming is kind of hard: is it clear that getRingColorString is not the getter associated with the setter setRingColor?