RFR: 8331616: ChangeListener is not triggered when the InvalidationListener is removed

[John Hendrikx]

This PR provides a fix for the linked issue.

The issue was that when an invalidation listener is removed, and the `ExpressionHelper` type changes from `Generic` to `SingleChange` that it would copy the current value of the `Generic` instance before it was updated (this is because invalidation listeners trigger before change listeners and the current value would only be updated **after** invalidation listeners notifications were completed).

The code now will update the current value before sending out any notifications **if** there are change listeners present to head off this problem.

Added a few test cases to verify this.

Note: the PR which replaces `ExpressionHelper` does not have this problem: https://github.com/openjdk/jfx/pull/1081

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Commit messages:
 - Fix problem in ExpressionHelper that could copy the wrong current value

Changes: https://git.openjdk.org/jfx/pull/1448/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1448&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331616
  Stats: 69 lines in 2 files changed: 66 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1448.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1448/head:pull/1448

PR: https://git.openjdk.org/jfx/pull/1448