RFR: JDK-8312528: Move Subscription interface from javafx.beans to javafx.util

[John Hendrikx]

The Subscription interface is independent from FX beans and has uses unrelated to beans, it should therefore be placed in javafx.util.

This should be fixed before [JDK-8311123](https://bugs.openjdk.org/browse/JDK-8311123) is released.

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Commit messages:
 - Move Subscription to javafx.util

Changes: https://git.openjdk.org/jfx/pull/1182/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1182&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312528
  Stats: 20 lines in 12 files changed: 9 ins; 7 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1182.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1182/head:pull/1182

PR: https://git.openjdk.org/jfx/pull/1182