RFR: 8370140: RichTextArea: line endings [v12]

[Andy Goryachev]

> Adds control of line endings (newline separators) in `StyledTextModel`, `RichTextArea`, and `CodeArea`.
> 
> The impacted areas are:
> - saving to plain text
> - copying to plain text
> - IME
> 
> This feature is implemented as a regular field in the `StyledTextModel` (since it is ultimately an attribute of the model), with convenience setter and getter in the `RichTextArea`.
> 
> ### NOTES
> 
> - some dependency on #1938 , resolved.

Andy Goryachev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:

 - Merge remote-tracking branch 'origin/master' into 8370140.line.separator
 - review comments
 - removed property
 - review comments
 - code area test
 - Merge branch 'master' into 8370140.line.separator
 - fixed tests
 - non-nullable
 - Merge branch 'master' into 8370140.line.separator
 - get text
 - ... and 6 more: https://git.openjdk.org/jfx/compare/5684edf7...285cf40e

-------------

Changes: https://git.openjdk.org/jfx/pull/1944/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1944&range=11
  Stats: 297 lines in 13 files changed: 265 ins; 11 del; 21 mod
  Patch: https://git.openjdk.org/jfx/pull/1944.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1944/head:pull/1944

PR: https://git.openjdk.org/jfx/pull/1944