Need an event for when a tab newly becomes visible, e.g., Show

[rcoreilly]

Describe the feature

@kkoreilly we have discussed this but I don't remember the current status. In the emer sims, we really need to be able to get the current state of the Network when someone switches back to that tab. Definitely do not want tab updating when not visible as it is expensive.

It would make sense for this to be the Show event, although there was some issue about whether Show is a "once" only event or not (could always use sync.Once to ensure where needed, presumably).

Relevant code

No response