Improve plot coloring

- the color picker goes on a different action (as in other geppetto applications)
- colors are assigned randomly at start
- the color picker is not appearing anymore when plotting
- in the plot, add a button to change the color of every plot. This can be a menu icon together with the other plot icons, but it must be implemented through a redux action (what about a parameter on the plot component to add personalized action buttons?).

Start by tracing the plot component and the existing color picker behavior, then inspect how plot icons and Redux actions are wired. Done means the picker appears during plotting, colors are assigned as requested, and a plot action can change every plot's color.

Written by the indexing model from the issue text.