RFR: 8357393: RichTextArea: fails to properly save text attributes

[Andy Goryachev]

Fixing a bug that breaks proper serialization of character attributes.

This, unfortunately, makes a breaking change in the RichTextArea native format [0].

## References

[0] https://github.com/andy-goryachev-oracle/Test/blob/main/doc/RichTextArea/RichTextArea_DataFormat_V2.md

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Commit messages:
 - javadoc
 - Merge remote-tracking branch 'origin/master' into 8357393.attr.ser
 - test
 - tests
 - fixed attribute serialization

Changes: https://git.openjdk.org/jfx/pull/1813/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1813&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357393
  Stats: 188 lines in 6 files changed: 96 ins; 41 del; 51 mod
  Patch: https://git.openjdk.org/jfx/pull/1813.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1813/head:pull/1813

PR: https://git.openjdk.org/jfx/pull/1813