Meshcat could have a list of objects monitoring their load

[Vinggui]

Since ThreeJS provides some some extra callbacks in its loaders, Meshcat could implement some list of objects that are incremented by set_object. This list can hold a loading status, which would give us more control while waiting for a complete scene load.

Currently, I would use this feature to monitor individual glTF objects that seems to take longer to load and trigger an event once they are ready.