RFR: 8348736: RichTextArea Follow-up 2

[Andy Goryachev]

Fixes minor issues uncovered while writing API tests #1677:

- StyledTextModel::clamp L559 is incorrect at the end of paragraph
- CodeArea.getText() adds extra newline at the end

This change should be back-ported to jfx24.

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Commit messages:
 - newlines
 - check
 - cleanup
 - clamp
 - get text

Changes: https://git.openjdk.org/jfx/pull/1684/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1684&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348736
  Stats: 214 lines in 7 files changed: 151 ins; 51 del; 12 mod
  Patch: https://git.openjdk.org/jfx/pull/1684.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1684/head:pull/1684

PR: https://git.openjdk.org/jfx/pull/1684