Add loading indicator

[caseyyee]

Some scene objects (like models) with lots of geometry or texture data take a while to load. It would be nice to have some kind of loading indicator so that the user knows that the scene is still loading that would otherwise look broken.

This scene particularly takes long to load:

[ngokevin]

dupe #423

[ngokevin]

if you're using dev branch, it should be perceivably faster.

[ngokevin]

#925