RFR: 8277000: Tree-/TableRowSkin: replace listener to fixedCellSize by live lookup

[Marius Hanl]

This PR improves the `Tree-/TableRowSkin` code by doing a normal live lookup for the `fixedCellSize` instead of adding listener just to update variables(`fixedCellSizeEnabled` and `fixedCellSize`) which can otherwise be also just lookup'd without the hassle of listeners.

While this is mostly a cleanup, it does improve the state of the `Tree-/TableRow`, as when the `TableRowSkinBase` constructor is called, the variables are not yet set.

It is also consistent with the other cells, see also [JDK-8246745](https://bugs.openjdk.org/browse/JDK-8246745).
Helps a bit with [JDK-8185887](https://bugs.openjdk.org/browse/JDK-8185887) (https://github.com/openjdk/jfx/pull/1644), but as written above, not required as there is no (visible) effect.

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Commit messages:
 - 8277000: Tree-/TableRowSkin: replace listener to fixedCellSize by live lookup

Changes: https://git.openjdk.org/jfx/pull/1645/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1645&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8277000
  Stats: 140 lines in 5 files changed: 78 ins; 44 del; 18 mod
  Patch: https://git.openjdk.org/jfx/pull/1645.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1645/head:pull/1645

PR: https://git.openjdk.org/jfx/pull/1645