RFR: JDK-8320444: Column drag header is positioned wrong for nested columns

[Marius Hanl]

When a nested column is dragged, the column drag header (blue) is positioned wrong.
It appears at the very left of the table and not where the column is.

![image](https://github.com/openjdk/jfx/assets/66004280/6c2a0f61-f32d-4c38-9549-e2e20f5bd4f8)

The fix is to calculate the bound based off the `TableHeaderRow` (and not the parent `NestedTableColumnHeader`.

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Commit messages:
 - JDK-8320444: Column drag header is positioned wrong for nested columns

Changes: https://git.openjdk.org/jfx/pull/1292/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1292&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320444
  Stats: 78 lines in 3 files changed: 77 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1292.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1292/head:pull/1292

PR: https://git.openjdk.org/jfx/pull/1292