RFR: 8087555: [ChoiceBox] uncontained value not shown

[Jeanette Winzenburg]

The issue is that ChoiceBoxSkin
a) doesn't update the text of the label if the value is not contained in the items
b) doesn't respect converter for label text

Fixed by
- listening to value changes to update the label
- removing ad-hoc updates (not needed), added update on converter change
- passing all label updates through converter

Added test for text updates that failed before the fix and pass after (note: there were no tests for the display text,
so for coveragy, contains also tests that passed before as well as after)

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Commit messages:
 - 8087555: [ChoiceBox] uncontained value not shown

Changes: https://git.openjdk.java.net/jfx/pull/191/files
 Webrev: https://webrevs.openjdk.java.net/jfx/191/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8087555
  Stats: 464 lines in 2 files changed: 451 ins; 8 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/191.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/191/head:pull/191

PR: https://git.openjdk.java.net/jfx/pull/191