RFR: 8285197: TableColumnHeader: calc of cell width must respect row styling (TreeTableView)

[Robert Lichtenberger]

Separate test class added for TreeTableView case.
Fix is analogous to JDK-8251480.

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Commit messages:
 - 8285197: TableColumnHeader: calc of cell width must respect row styling (TreeTableView)

Changes: https://git.openjdk.java.net/jfx/pull/779/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=779&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285197
  Stats: 183 lines in 2 files changed: 179 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/779.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/779/head:pull/779

PR: https://git.openjdk.java.net/jfx/pull/779