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

[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 item list. Fixed the issue by restoring the selection in this case.

Added a unit test to validate the fix

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Commit messages:
 - Fix item selection issue

Changes: https://git.openjdk.org/jfx/pull/1452/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1452&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8279140
  Stats: 33 lines in 2 files changed: 33 ins; 0 del; 0 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