xilem_web: Add external communication example

[flosse]

I repeatedly had problems combining xilem_web with "external systems".
Here is an attempt to establish a connection via channels.
It still feels a bit awkward, but at least it is possible without modifications to xilem_web.
If there are more elegant approaches, I would be very interested ☺️

[PoignardAzur]

Thanks for the PR!

Overall I think we're likely to move to an architecture similar to this, where apps have a root "reactor" task that starts background threads and dispatches messages.

I'm not sure we should add this as an example, though. I think the Xilem repository already has too many "proof of concepts" examples that show a potential architecture but don't do anything concrete.