RFR: 8356770: TreeTableView not updated after removing a TreeItem with children and adding it to another parent [v4]

[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 change

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1971/files
  - new: https://git.openjdk.org/jfx/pull/1971/files/58f9f4e7..e160a15a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1971&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1971&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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