RFR: JDK-8325402: TreeTableRow updateItem() does not check item with isItemChanged(..) unlike all other cell implementations [v2]

[Marius Hanl]

> `TreeTableRow` does not check the item with `isItemChanged(..)`, unlike all other implementations of the cell.
> 
> This also means that the `TreeTableRow` always updates the item, although it should not, resulting in a performance loss as a `TreeTableRow` will always call `updateItem(..)`.
> 
> It looks like that this was forgotten in [JDK-8092593](https://bugs.openjdk.org/browse/JDK-8092593).
> 
> Checking the whole history, it looks like the following was happening:
> 1. There was a check if the item is the same in all cell implementations (with `.equals(..)`)
> 2. Check was removed as it caused bugs
> 3. Check was readded, but instead we first check the index (same index) and then if we also have the same item (this time with `.isItemChanged(..)`, to allow developers to subclass it)
> 4. Forgotten for `TreeTableRow` inbetween this chaos.
> 
> Added many tests for the case where an item should be changed and where it should not.
> Improved existing tests as well. Using `stageLoader`, as before the tests only created a stage when doing the assert.
> 
> NOTE: The update logic is now the same as for all other 5 cell implementations. Especially `TreeCell` (since it is for a tree structure as well).

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

 - JDK-8325402: remove labeled if
 - JDK-8325402: Unit test improvements

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1360/files
  - new: https://git.openjdk.org/jfx/pull/1360/files/947b4153..a7584d81

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1360&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1360&range=00-01

  Stats: 29 lines in 10 files changed: 11 ins; 12 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1360.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1360/head:pull/1360

PR: https://git.openjdk.org/jfx/pull/1360