Use a Service Worker to keep connection alive while dialog is open or browser is in background

[rodja]

As we discussed in #2286, for truly non-blocking execution in JavaScript we should sepearate the connection to the server from the main execution thread. A simple example to reproduce the problem is

label = ui.label()
ui.timer(0.1, lambda: label.set_text(f'{time.time():.3f}'))
ui.button('Print', on_click=lambda: ui.run_javascript('print()'))

The time does not get updated anymore as long as the print dialog is opened. That leads to a connection loss when reconnect_timeout is reached.

[rodja]

I guess #2073 suffered from this. And maybe #2715, too.

[BaccanoMob]

Also #4881 (js script to prevent page refesh/reload by warning in alert dialog)

[evnchn]

Got #5784