RFR: 8356770: TreeTableView not updated after removing a TreeItem with children and adding it to another parent [v3]

[Ziad El Midaoui]

> When a subtree is moved in a `TreeTableView` , the visuals don’t update until a resize or expand/collapse. The `TreeTableViewSkin` only rebuilds cells when the expanded row count changes.
> This PR makes the skin to detect structural changes on `childrenModificationEvent` using new variable `treeStructureDirty`, and in `updateItemCount()` call `requestRebuildCells()` to refresh the visuals.

Ziad El Midaoui has updated the pull request incrementally with one additional commit since the last revision:

  Minor test changes

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1971/files
  - new: https://git.openjdk.org/jfx/pull/1971/files/4a07d87e..58f9f4e7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1971&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1971&range=01-02

  Stats: 29 lines in 2 files changed: 4 ins; 18 del; 7 mod
  Patch: https://git.openjdk.org/jfx/pull/1971.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1971/head:pull/1971

PR: https://git.openjdk.org/jfx/pull/1971