RFR: 8189354: ArrayIndexOutOfBoundsException when listening to selection changes on TreeTableView [v7]

[mstr2]

> This PR fixes the implementation of `ControlUtils.reducingChange`, which incorrectly computed adjacent removed indices, thus resulting in incorrect removal notifications.
> 
> Since there were no unit tests for this method, I also added a bunch of tests.
> 
> After applying this fix, I can no longer reproduce [JDK-8189354](https://bugs.openjdk.java.net/browse/JDK-8189354) and [JDK-8189228](https://bugs.openjdk.java.net/browse/JDK-8189228).

mstr2 has updated the pull request incrementally with one additional commit since the last revision:

  Use MockListObserver for testing

-------------

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/480/files
  - new: https://git.openjdk.java.net/jfx/pull/480/files/1f7e2717..4b9ae2a6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=480&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=480&range=05-06

  Stats: 42 lines in 2 files changed: 26 ins; 9 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/480.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/480/head:pull/480

PR: https://git.openjdk.java.net/jfx/pull/480