RFR: 8252236: TabPane - initial select tab not working properly

[Jeanette Winzenburg]

the issue is that the header of the selected tab is not always visible (or kept visible, see report for details)

- initially
- after changing side
- after resizing stage/tabPane

Fixed in TabPaneSkin to notify its TabHeaderArea (== collaborator that is responsible for layout the tabs) whenever its
scroll might need an update.

Added tests that fail before and pass after the fix.

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Commit messages:
 - 8252236: TabPane - initial select tab not working properly

Changes: https://git.openjdk.java.net/jfx/pull/300/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=300&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252236
  Stats: 361 lines in 3 files changed: 353 ins; 3 del; 5 mod
  Patch: https://git.openjdk.java.net/jfx/pull/300.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/300/head:pull/300

PR: https://git.openjdk.java.net/jfx/pull/300