[jfx24] RFR: 8348736: RichTextArea clamp and getText

[Andy Goryachev]

Clean backport of RichTextArea clamp and getText fix to the jfx24 stabilization branch.

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Commit messages:
 - Backport 2b5b11320087de9b7bb6f330236a5c15eb4cabea

Changes: https://git.openjdk.org/jfx/pull/1688/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1688&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348736
  Stats: 220 lines in 7 files changed: 156 ins; 51 del; 13 mod
  Patch: https://git.openjdk.org/jfx/pull/1688.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1688/head:pull/1688

PR: https://git.openjdk.org/jfx/pull/1688