Static UI as assets

[JMS55]

Composing UI, especially reactive UI, as code is good. But for some types of games, it would be nice to generate static UI visually in the editor.

Open questions:

1. How do we map hand-written rsx syntax in an asset file -> dioxus UI element/templates at runtime?
2. Can we support defining the template by hand, and then passing in code-defined props and ECS-data-via-hooks to make the parts of the UI dynamic?
3. Do we need to hookup a VirtualDom to for asset-based UIs (assuming we don't hydrate the static asset with dynamic parts)? It might suffice to simply spawn the appropriate entities once, and skip setting up mutations.

[JMS55]

We can parse rust-code-as-a-string into rsx with syn::parse_str::<T>(), where T: BodyNode.