We should let scene files define entities with attached asset handles.

[oddfacade]

I have a draft implementation here (click here to view changes). Essentially, it defines a new kind of pseudo-"trait reflection" (kind of like ReflectComponent) that lets DynamicScene::write_to_world obtain components from objects that implement the Proxy trait. This is general enough that I can imagine doing something similar for things like nested scenes, inline assets, bundle loading, etc... and frankly that's the direction I'm hoping to take this... but as a sort of proof of concept I just made it do so-called ProxyHandles for now. They turn into strong handles when you spawn the scene.

If there's interest in this, I can write up an RFC or draft PR, but I just want to make sure I'm in the right ballpark here before I get in too deep; especially since I'm not entirely sure where things stand wrt #3972 and #92.