RFR: 8341440: ScrollPane: no immediate effect changing fitWidth/fitHeight

[Andy Goryachev]

- fixed the issue
- added unit tests
- clarified default value of ScrollPane.fitWidth and .fitHeight properties

No CSR is needed as it is a minor clarification.

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Commit messages:
 - 8341440: ScrollPane: no immediate effect changing fitWidth/fitHeight

Changes: https://git.openjdk.org/jfx/pull/1597/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1597&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341440
  Stats: 67 lines in 3 files changed: 66 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1597.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1597/head:pull/1597

PR: https://git.openjdk.org/jfx/pull/1597