[Rev 02] 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.

Ambarish Rapte has updated the pull request incrementally with one additional commit since the last revision:

  Fix review comment, add test

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/147/files
  - new: https://git.openjdk.java.net/jfx/pull/147/files/43624547..e81f175a

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/147/webrev.02
 - incr: https://webrevs.openjdk.java.net/jfx/147/webrev.01-02

  Stats: 27 lines in 2 files changed: 24 ins; 0 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