Ongoing cleanup

1. Don't default `ColorMaterialProperty` to an arbitrary value.
2. Improve `ColorMaterialProperty` constructor.
3. Clean up `viewerDynamicObjectMixin` after integrate from master.
4. Tweak GeoJsonDataSource default polygon fill
5. Clean up some doc.
Delete VisualizerCollection

It has been superceded by DataSourceDisplay and is no longer needed.