RFR: 8237453: [TabPane] Incorrect arrow key traversal through tabs after reordering

[Ambarish Rapte]

Issue: 
Dragging a tab and dropping it back to it's original position causes an incorrect reordering of tabs. Incorrect order of tabs can be observed using arrow key traversal through tabs.

Cause:
Below mechanism of identifying if tab header is dragged and dropped at same position is not reliable. 
When a tab is selected for dragging, the tab's layout X is stored as `dragHeaderStartX`.
When the tab is dropped, the tab's destination layout X is saved as `dragHeaderDestX`.
These two values are used to decide if the reordering of tabs is needed.
This is an unreliable mechanism in two cases,

1. When the `TabPane.TabClosingPolicy` is set to `TabPane.TabClosingPolicy.SELECTED_TAB`, the close button icon gets added only to the selected tab. 
Each time when the tab selection is changed, the header region needs a re-layout. When a non selected tab is chosen for dragging `dragHeaderStartX` and `dragHeaderDestX` will always differ.

2. Calculation of `dragHeaderDestX` involves floating point operation. So the computed `dragHeaderDestX` may sometimes vary by fraction than `dragHeaderStartX`.

Fix:
Use the index of tab header for identifying the tabs which are reordered.
Save the index when reordering begins. `dragTabHeaderStartIndex`
Save the index when tab is dropped. `dragTabHeaderIndex`
If the index are same then reordering of tabs is NOT needed.
If the index are NOT same then the tabs from `dragTabHeaderStartIndex` to `dragTabHeaderIndex` are reordered.

Verification:
This fix is specific to TabPane reordering. No other tests/behavior should be affected.
Added a new system test.

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

Commits:
 - 835e7aa8: [WIP]8237453: [TabPane] Incorrect arrow key traversal through tabs after reordering

Changes: https://git.openjdk.java.net/jfx/pull/106/files
 Webrev: https://webrevs.openjdk.java.net/jfx/106/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8237453
  Stats: 311 lines in 2 files changed: 300 ins; 4 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/106.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/106/head:pull/106

PR: https://git.openjdk.java.net/jfx/pull/106