RFR: 8252863: Spinner keeps spinning if removed from Scene [v3]

[Karthik P K]

> Spinner was not stopping because it was getting removed from scene before the mouse release event handler was getting invoked.
> 
> Added listener for `sceneProperty` so that when Spinner is removed from the scene, `stopSpinning` method shall be called.
> 
> Added unit test to validate the fix

Karthik P K has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.org/jfx/pull/976/files
  - new: https://git.openjdk.org/jfx/pull/976/files/6b59cbe8..deaf07ac

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=976&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=976&range=01-02

  Stats: 3 lines in 2 files changed: 2 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/976.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/976/head:pull/976

PR: https://git.openjdk.org/jfx/pull/976