RFR: 8236840: Memory leak when switching ButtonSkin

[Ambarish Rapte]

ButtonSkin adds a `ChangeListener` to `Control.sceneProperty()` which results in leaking the `ButtonSkin` itself when
the `Button`'s skin is changed to a new `ButtonSkin`.  Using a `WeakChangeListener` instead of `ChangeListener` solves
the issue.

Please take a look.

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Commit messages:
 - 8236840: Memory leak when switching ButtonSkin

Changes: https://git.openjdk.java.net/jfx/pull/147/files
 Webrev: https://webrevs.openjdk.java.net/jfx/147/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8236840
  Stats: 84 lines in 2 files changed: 64 ins; 17 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/147.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/147/head:pull/147

PR: https://git.openjdk.java.net/jfx/pull/147