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