RFR: JDK-8304439: Subscription based listeners [v13]

[Nir Lisker]

On Thu, 13 Jul 2023 00:01:23 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> Makes `Subscription` public (removing some of its methods that are unnecessary), and adds methods that can provide `Subscription`s in `ObservableValue`.
>
> John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix ObservableSubscriptionsTest

The tests look fine. Doc changes can be deferred.

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Marked as reviewed by nlisker (Reviewer).

PR Review: https://git.openjdk.org/jfx/pull/1069#pullrequestreview-1527552887