Add UIx demo

[ggeoffrey]

From @elken, ported from the legacy Electric Examples App. Original PR.

Add a demo for https://github.com/pitch-io/uix per discussion on slack

I wasn't able to start this up locally due to datomic, but I've tested the main code on the xtdb-starter and all functions as expected 😄

Don't know if this codebase is also subject to the same agreement as the main electric repo; if that's the case let me know on slack and I'll sign it 😄

Changes: add-uix-demo.patch

[ggeoffrey]

I merged the code from your original PR, adapting it to latest Electric Fiddle: commit

Thank you for your contribution!