Add wasm support to the `element` crate

[FredrikNoren]

The element crate is our own "React" implementation, which is able to render a tree of UI based on state. It currently only runs in the native context, but we could potentially make it support the wasm target too (and make it depend on our wasm api). If we do that, we could split up the ui crate into ui and ui_components, where ui would just be the native stuff, and ui_components would be supported on both native and wasm, and would contain things like Button etc.