RFR: 8340344: The first item in TreeView is not aligned in the beginning [v3]

[Ziad El Midaoui]

> The issue occurred because items preceding an item with children (items with a disclosure node) had different widths, which led to misalignment. This can be fixed by requesting a cell relayout whenever the disclosure node's width changes.

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

  Fixed minor issues and added test

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1715/files
  - new: https://git.openjdk.org/jfx/pull/1715/files/a07148a2..6ae2f74b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1715&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1715&range=01-02

  Stats: 54 lines in 2 files changed: 48 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/1715.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1715/head:pull/1715

PR: https://git.openjdk.org/jfx/pull/1715