RFR: JDK-8304439: Subscription based listeners [v3]

[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 three additional commits since the last revision:

 - Improve javadoc
 - Add copyright headers
 - Rename Subscription#of to Subscription#combine

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1069/files
  - new: https://git.openjdk.org/jfx/pull/1069/files/df31b3fa..23fe903f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1069&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1069&range=01-02

  Stats: 57 lines in 3 files changed: 52 ins; 0 del; 5 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