RFR: JDK-8305248: TableView not rendered correctly after column is made visible if fixed cell size is set [v2]

[Marius Hanl]

> The determined `prefWidth` of a `TableCell` could be `0.0` when a `fixedCellSize` is set.
> This happened because the `TableCell` may not have a skin since it was never added to the scene graph yet.
> 
> The fix is to make sure we get the `prefWidth` after the `TableCell` was added to the scene graph.
> That is also the reason why the problem only happened the first time and never again after (skin is then already created).

Marius Hanl has updated the pull request incrementally with two additional commits since the last revision:

 - JDK-8305248: Added the tests also for TreeTableRow
 - JDK-8305248: Improve comments

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1077/files
  - new: https://git.openjdk.org/jfx/pull/1077/files/a53ffaa9..703115c5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1077&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1077&range=00-01

  Stats: 72 lines in 3 files changed: 57 ins; 5 del; 10 mod
  Patch: https://git.openjdk.org/jfx/pull/1077.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1077/head:pull/1077

PR: https://git.openjdk.org/jfx/pull/1077