RFR: 8252863: Spinner keeps spinning if removed from Scene

[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

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Commit messages:
 - Remove commented line
 - Add removed empty line
 - Fix spinner running issue on removing from scene

Changes: https://git.openjdk.org/jfx/pull/976/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=976&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8252863
  Stats: 115 lines in 5 files changed: 112 ins; 0 del; 3 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