RFR: 8295175: SplitPaneSkin: memory leak when changing skin

[Andy Goryachev]

Fixed memory leak by removing all the listeners in dispose();

This PR depends on a new internal class ListenerHelper, a replacement for LambdaMultiplePropertyChangeListenerHandler.
See https://github.com/openjdk/jfx/pull/908

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Commit messages:
 - Merge remote-tracking branch 'origin/master' into
 - Merge branch '8294809.listener.helper' into 8295175.splitpaneskin.with.helper
 - 8294809: generics
 - Merge branch '8294809.listener.helper' into 8295175.splitpaneskin.with.helper
 - 8294809: is alive
 - Merge branch '8294809.listener.helper' into 8295175.splitpaneskin.with.helper
 - Revert "8294809: removed weak listeners support"
 - Merge branch '8294809.listener.helper' into 8295175.splitpaneskin.with.helper
 - 8294809: removed weak listeners support
 - 8295175: rebuild dividers
 - ... and 16 more: https://git.openjdk.org/jfx/compare/3376228a...97995ec8

Changes: https://git.openjdk.org/jfx/pull/911/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=911&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295175
  Stats: 70 lines in 2 files changed: 31 ins; 18 del; 21 mod
  Patch: https://git.openjdk.org/jfx/pull/911.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/911/head:pull/911

PR: https://git.openjdk.org/jfx/pull/911