Update UI in Billboards, Labels and Camera Sandcastle examples

For #1685 

Updated UI in Billboards, Labels, Camera (converted buttons to combobox)
Re-positioned buttons in Materials
Updated a few outdated pictures
Added GeoJSON/TopoJSON example
@mramato this is separate from this PR, but does `dataSources.add` return a data source like `primitives.add` and would a helper like `Model.fromGltf` help here to do all this in one line of code?