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.
**WARNING:** This PR will probably blow up all in-flight PRs
at some point in the early days of CmdPal, two of us created seperate
`Exts` and `exts` dirs. Depending on what the casing was on the branch
that you checked one of those out from, it'd get stuck like that on your
PC forever.
Windows didn't care, so we never noticed.
But GitHub does care, and now browsing the source on GitHub is basically
impossible.
Closes#38081
