Consider Refactoring link.rs

[terhechte]

The link is a bit of a terrible abstraction to handle background threads via egui. The way it works is that when an action should be performed in the background, it will send the action over a crossbeam channel to a separate thread which continuously listens for input from the channel. The thread processes the action (e.g. performs a query) and sends it back.

Egui, in the default event loop, will check for any response in the receiver crossbeam channel (e.g. 30 times per second on 30fps). If a response is available it will be applied.

There're a couple of downsides to this:

• Request and Response are separated which makes it very difficult to use the Engine via Link in a non-async context.
• WASM doesn't support threads, so there is a wonky abstraction to allow sync access (with way too many cfg(target_arch = wasm32)
• This solution requires multiple additional abstractions which make it all a bit confusing to understand.

Egui apparently would work from separate threads but this might not work for other UI libraries (if we ever port it). So maybe a solution would be to have a SyncLink and AsyncLink with a closure that makes sure the request matches the response.