RFR: 8295242: ScrollBarSkin: memory leak when changing skin

[Andy Goryachev]

Fixed memory leak when changing skin in ScrollBarSkin (was adding an event handler without removing it in dispose()).

This change depends on ListenerHelper PR
https://github.com/openjdk/jfx/pull/908

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Commit messages:
 - 8295242: cleanup
 - Merge remote-tracking branch 'origin/master' into 8295242.scrollbar.skin
 - Merge branch '8294809.listener.helper' into 8295242.scrollbar.skin
 - 8294809: generics
 - Merge branch '8294809.listener.helper' into 8295242.scrollbar.skin
 - 8294809: is alive
 - Revert "8294809: removed weak listeners support"
 - 8295242: cleanup
 - Merge branch '8294809.listener.helper' into 8295242.scrollbar.skin
 - 8294809: removed weak listeners support
 - ... and 15 more: https://git.openjdk.org/jfx/compare/3376228a...dd27e686

Changes: https://git.openjdk.org/jfx/pull/914/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=914&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295242
  Stats: 29 lines in 2 files changed: 7 ins; 9 del; 13 mod
  Patch: https://git.openjdk.org/jfx/pull/914.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/914/head:pull/914

PR: https://git.openjdk.org/jfx/pull/914