Element highlighting causes BulletChart DataPoint and Legend color read failures

[mshriver]

When element highlighting is enabled, the DataPoint.color and Legend.color can end up reading rgb(0, 0, 0). This is due to the highlighting DOM change making pie elements go black in color instead of display a red border.

This causes unit test failures, and will impact users of the element highlighting feature in general.

I would propose making the Legend class implement properties for label, value, and color, which will use the element instance attribute to make a lazy lookup.

A potential change to the Legend class that would make sense to do concurrently would be convert to a Widget class and use a ParametrizedLocator instead of passing the element from BulletChart on instantiation.

See #159 as a recreation.

[digitronik]

It fixed. please reopen it if needed.