RFR: 8295796: ScrollPaneSkin: memory leak when changing skin

[Andy Goryachev]

as determined by SkinMemoryLeakTest (remove line 174) and a leak tester 
https://github.com/andy-goryachev-oracle/Test/blob/main/src/goryachev/apps/LeakTest.java

caused by:
- adding and not removing listeners
- adding and not removing event handlers/filters

NOTE:
this fix requires JDK-8295242 scroll bar fixes, as well as JDK-8294809 ListenerHelper

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Commit messages:
 - 8295796: whitespace
 - Merge remote-tracking branch 'origin/master' into
 - 8295796: ScrollPaneSkin: memory leak when changing skin
 - Merge remote-tracking branch 'origin/master' into 8295796.scroll.pane.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
 - ... and 17 more: https://git.openjdk.org/jfx/compare/3376228a...3e9678a9

Changes: https://git.openjdk.org/jfx/pull/928/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=928&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295796
  Stats: 107 lines in 3 files changed: 23 ins; 26 del; 58 mod
  Patch: https://git.openjdk.org/jfx/pull/928.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/928/head:pull/928

PR: https://git.openjdk.org/jfx/pull/928