RFR: JDK-8304439: Subscription based listeners

[John Hendrikx]

Makes `Subscription` public (removing some of its methods that are unnecessary), and adds methods that can provide `Subscription`s in `ObservableValue`.

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Commit messages:
 - Make Subscription public and add methods to provide them

Changes: https://git.openjdk.org/jfx/pull/1069/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1069&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304439
  Stats: 461 lines in 11 files changed: 328 ins; 122 del; 11 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