RFR: JDK-8224260: ChangeListener not triggered when adding a new listener in invalidated method [v2]

[John Hendrikx]

> Fixes three issues in ExpressionHelper:
> 
> - Current Value was not retained when changing from SingleChange to Generic, this can lead to missed changes
> - Current Value was not retained when changing from Generic to SingleChange, this can lead to missed changes
> - Current Value was not cleared when last change listener was removed in Generic variant, resulting in an older value being referenced and not becoming eligible for GC until either a ChangeListener is added again, or sufficient InvalidationListeners are removed to switch to the SingleInvalidation implementation...

John Hendrikx has updated the pull request incrementally with one additional commit since the last revision:

  Doc improvements

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1078/files
  - new: https://git.openjdk.org/jfx/pull/1078/files/8fb56715..c4cb0c12

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1078&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1078&range=00-01

  Stats: 15 lines in 2 files changed: 11 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1078.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1078/head:pull/1078

PR: https://git.openjdk.org/jfx/pull/1078