Committing this sample for posterity's sake.
This is the product of an experiment: "can an extension know when it's
page was opened / closed?"
And without changing the actual SDK, this **is** possible. It relies on
the fact that CmdPal (the host) needs to register for the `ItemsChanged`
event on list pages, and it does that when the page is loaded, and it
unregisters itself when the page is closed.
This does require manually implementing `IListPage` - you can't just
`override` the `event` in the base class ([that's
illegal](https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/quality-rules/ca1070)),
and using `new` to shadow it doesn't work either (probably for cswinrt
reasons).
This is the best I came up with.
