WebUI ping for inactive tabs

[mcspr]

ref. #2544

Annoying window saying "Connection lost with the device, click OK to refresh the page" after 5 minutes of idleness.
Firefox cheeky switches to this tab every 5 minutes! (In version 1.14, there were no such problems at all)

subj.

• https://developer.mozilla.org/en-US/docs/Web/API/Window/pageshow_event
• https://developer.mozilla.org/en-US/docs/Web/API/Window/pagehide_event
• https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API#policies_in_place_to_aid_background_page_performance

Connection itself seems to be protected, only timers are an issue.

Tabs running code that's using real-time network connections (WebSockets and WebRTC) go unthrottled in order to avoid closing these connections timing out and getting unexpectedly closed.

Chrome supposedly does throttling on timers as well

• https://blog.chromium.org/2020/11/tab-throttling-and-more-performance.html

Beginning in M87, we’re throttling JavaScript timer wake-ups in background tabs to once per minute.

Mobile versions are expected to do even more :)