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

[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.

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Commit messages:
 - Fixed import error
 - Merge branch 'openjdk:master' into 8340344.TreeViewAlign
 - Fix by explicitly requesting layout on cells instead of changing default value
 - Merge branch 'openjdk:master' into 8340344.TreeViewAlign
 - Fixed default disclosure width

Changes: https://git.openjdk.org/jfx/pull/1715/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1715&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340344
  Stats: 19 lines in 1 file changed: 19 ins; 0 del; 0 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