[jfx26] RFR: 8373936: RichEditorDemoApp enhancements

[Andy Goryachev]

Backport of the following change:

[JDK-8376138](https://bugs.openjdk.org/browse/JDK-8376138) RichTextArea: getModelStyleAttrs might return wrong attributes

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Commit messages:
 - Backport 744d7fa3c9a69542924043802db6422bce2e184d

Changes: https://git.openjdk.org/jfx/pull/2057/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2057&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373936
  Stats: 1323 lines in 12 files changed: 846 ins; 349 del; 128 mod
  Patch: https://git.openjdk.org/jfx/pull/2057.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2057/head:pull/2057

PR: https://git.openjdk.org/jfx/pull/2057