RFR: 8290040: Provide simplified deterministic way to manage listeners [v9]

[John Hendrikx]

On Fri, 2 Dec 2022 00:34:35 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> The API and docs for `ObservableValue::when` look like they are in good shape, so I'll review them next week.
> 
> My only overall comment is that this PR should be limited to the new API, implementation, and tests in `javafx.base`. Although it's helpful to review them together, the changes to `Node` to add a new `shown` property is an interesting use case that should stand on its own merits and should be done as a follow-on Enhancement in a separate PR.

Okay, that would mean removing the examples from `when` that use the `shownProperty`, and I suppose adding them back in later.  The ticket was specifically worded to make it easier to deterministically manage listeners, which at least for UI controls, the `shown` property is an important part of.

If you think its realistic, I can file a new ticket, split it off so hopefully both can be part of the same release?

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PR: https://git.openjdk.org/jfx/pull/830