add verticalBarSetColors

{sorry for the duplicate, i pull-requested master instead of the feature branch}

updates the barColor and barBackgroundColor in the BarConfig of a VeticalBar.
repaints the bar if it its bootstrapped, or just update cfg for next time

useful for a volume bar that turns red on mute.

i also use it to depict values greater than 1.0. i change the color {from, say, green-on-black to blue-on-green} and subtract 1 from the value {and so on until the value is <= 1.0}.
this makes sense in the context of volume, for indicating amplification without wasting space.

i would greatly appreciate if you merged it, but barring that, could you instead simply export the barstate field accessors?