glTF with multiple scenes only draws default scene

[Sean Curtis]

When opening a glTF scene, the viewer silently views only the default scene. There are a number of better behaviors (in increasing order of "better"):

• It should inform you that there are multiple scenes and the default is being loaded.
• It should list the scenes and give you an option which one you'd like to load.
• It should give you the option to merge the scenes (or pick some subset of the scenes to load simultaneously).

This will probably require interaction with meshcat.