[Rev 01] 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:

  Changes for Kevin's review comment

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/147/files
  - new: https://git.openjdk.java.net/jfx/pull/147/files/d393e81d..43624547

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/147/webrev.01
 - incr: https://webrevs.openjdk.java.net/jfx/147/webrev.00-01

  Stats: 56 lines in 2 files changed: 52 ins; 0 del; 4 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