RFR: JDK-8304439: Subscription based listeners [v9]

[John Hendrikx]

> 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:

  Extend tests to test unsubscribe

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1069/files
  - new: https://git.openjdk.org/jfx/pull/1069/files/c297afbe..525c7841

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1069&range=08
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1069&range=07-08

  Stats: 32 lines in 2 files changed: 26 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1069.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1069/head:pull/1069

PR: https://git.openjdk.org/jfx/pull/1069