RFR: 8366201: RichTextArea: remove allowUndo parameter [v6]

[Andy Goryachev]

> Original user feedback (see https://mail.openjdk.org/pipermail/openjfx-discuss/2025-August/000267.html ) called for adding an `allowUndo` parameter to `applyStyle()` and `setStyle()` methods similarly to `replaceText()`.
> 
> Upon further analysis, the `allowUndo` parameter was a mistake: allowing the application code to disable creating undo/redo entries messes up the internal undo/redo stack.
> There is an internal need (`UndoableChange`), but it should not be exposed via public API.
> 
> This PR also adds `isUndoRedoEnabled()` and `setUndoRedoEnabled()` to the `StyledTextModel`, as well as its forwarding aliases to `RichTextArea` to allow for the application to disable undo/redo temporarily, for example, when building a document from multiple segments.
> 
> WARNING this is an incompatible change, permitted because of the incubator.
> 
> There remains a possible issue with currently unlimited size of the undo/redo stack - perhaps we should limit its depth to maybe 100-200 entries, see https://bugs.openjdk.org/browse/JDK-8370447 .

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

  javadoc

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1941/files
  - new: https://git.openjdk.org/jfx/pull/1941/files/361088e5..187678ff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1941&range=05
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1941&range=04-05

  Stats: 12 lines in 1 file changed: 8 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1941.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1941/head:pull/1941

PR: https://git.openjdk.org/jfx/pull/1941