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

• [x] Visiblity toggle

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

[PaulHax]

Attempt here #670