RFR: 8371183: RichTextModel: ContentChange.isEdit incorrect undoing style modification [v2]

[Andy Goryachev]

> Fixes an issue found by an external user:
> 
> Undoing a style only change fires a change event where `ContentChange.isEdit` is true when it should be false.
> 
> NOTE
> 
> - this PR depends on #1941 (resolved)

Andy Goryachev has updated the pull request incrementally with one additional commit since the last revision:

  bad merge

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1961/files
  - new: https://git.openjdk.org/jfx/pull/1961/files/49c78747..9162e0af

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1961&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1961&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1961.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1961/head:pull/1961

PR: https://git.openjdk.org/jfx/pull/1961