RFR: 8279140: ComboBox can lose selected value on item change via setAll [v2]

[Karthik P K]

> The `ComboBox` value was not set to previously selected value in the item list change listener when `setAll` method is used to change the items. Fixed the issue by restoring the selection in this case.
> 
> Added a unit test to validate the fix.
> [JDK-8279139](https://bugs.openjdk.org/browse/JDK-8279139) is also fixed with this change and added a unit test for the same.

Karthik P K has updated the pull request incrementally with one additional commit since the last revision:

  Fix JDK-8279139

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1452/files
  - new: https://git.openjdk.org/jfx/pull/1452/files/0f362fd7..f1582a2f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1452&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1452&range=00-01

  Stats: 41 lines in 2 files changed: 33 ins; 6 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1452.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1452/head:pull/1452

PR: https://git.openjdk.org/jfx/pull/1452