RFR: 8372298: RichTextArea: missing API: applyParagraphStyle

[Andy Goryachev]

A missing protected method in StyledTextModel prevents from toggling of an individual paragraph attribute.

This can be tested with he latest Monkey Tester
https://github.com/andy-goryachev-oracle/MonkeyTest

- select the RichTextArea page
- select the RichTextArea model
- select Control -> Context Menu -> RichTextArea menu
- type in several paragraphs
- select all and apply one paragraph attribute, let's say Paragraph Styles -> Background -> (any non-null)
- select a subset of previously selected paragraphs and apply a second paragraph attribute, let's say Paragraph Styles -> Bullet -> (any non-null)

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Commit messages:
 - test
 - fix

Changes: https://git.openjdk.org/jfx/pull/1980/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1980&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372298
  Stats: 94 lines in 8 files changed: 87 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jfx/pull/1980.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1980/head:pull/1980

PR: https://git.openjdk.org/jfx/pull/1980