Segment Group Stroke and Fill rendering

[PaulHax]

For segment groups, folks want a stroke outline so they can disable the segment fill to see image better.

GUI for each segment:

• Slider for stroke thickness
• Slider for fill transparency
• Slider for stroke transparency
• Visiblity toggle

rendering https://kitware.github.io/vtk-js/examples/ImageLabelOutline.html

[PaulHax]

Attempt here #670