Full or partial JSON export utils

[vincentfretin]

I'm using a json export/import in my project, implementation that I also proposed in 3dstreet in 3DStreet/3dstreet#537
Put that code in this repo. That's just some functions, there is no UI here.
That json format can be used for:

• Create a simple backend to save and share scenes #27
• Pasting the scene and/or entities from any other places right into the editor #24

Depends on

• Add new entityreparent command and ui to reorder / reparent an entity #26 (for the objectToElement and elementToObject helper functions)