Add a “New Update Available” notification

[fretje]

See this blog post:

https://whuysentruit.medium.com/blazor-wasm-pwa-adding-a-new-update-available-notification-d9f65c4ad13

I think this is something that's very interesting to have... I'll definitely build it in once I find the time... but thought this might be a "good first issue" for any takers? ;-)