RFR: 8252236: TabPane: must keep header of selected tab visible [v3]

[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
> - after modifying the list of tabs
> 
> 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.
> 
> Note that there still are issues while/after dragging, which are separated into
> [JDK-8253352](https://bugs.openjdk.java.net/browse/JDK-8253352)

Jeanette Winzenburg has updated the pull request incrementally with one additional commit since the last revision:

  removed empty else-block and added code comment

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/300/files
  - new: https://git.openjdk.java.net/jfx/pull/300/files/c3583829..9b95e0f6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=300&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=300&range=01-02

  Stats: 2 lines in 1 file changed: 1 ins; 1 del; 0 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