RFR: 8356770: TreeTableView not updated after removing a TreeItem with children and adding it to another parent [v2]

[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:

  Add test

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1971/files
  - new: https://git.openjdk.org/jfx/pull/1971/files/13d8b77b..4a07d87e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1971&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1971&range=00-01

  Stats: 64 lines in 1 file changed: 64 ins; 0 del; 0 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