Merge dev-* branches

[marceline-cramer]

Right now we have a bunch of dev-<something> branches pushed that have different features. At the time of writing, these are:

• dev-flycam: an interactive first-person 3D flycam
• dev-gltf: a glTF model loader
• dev-ui: a prototype of a UI system
• dev-physics: an extensible physics system as a service.

We're waiting on #260 to implement the code that will let us plumb these services together in a way that doesn't conflict with the UX. After that issue is resolved, we should work through integrating each of those branches into the codebase as useful services that we can use later.