Core separation

This PR looks non-mergeable, but that's fine for now. The bordeaux3d-core API is defined in the front/src/index.js file. The API isn't fully implemented yet, but I can see in the `core-client.html` file that buildings are loading and displaying and FirstPersonControl sort-of works which is a good first step towards demonstrating embeddability and reusability of core-only. 

Important changes :
* The metadata doesn't need to be inlined in the HTML any longer (for now, it's loaded from the server via an HTTP request. The metadata server-side API needs to be written)
* Moved onCameraViewChangeFirstPerson to FirstPersonControls (where it should have belonged in the first place)
* A container can be provided (instead of `document.body` being hardcoded)

Commits which make the PR mergeable to follow