RFR: 8366201: RichTextArea: remove allowUndo parameter

[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.

Whenever the application needs to disable undo/redo functionality (while, for example, building a document from multiple segments), this can be accomplished by calling clearUndoRedo().

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 (in a follow-up ticket).

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Commit messages:
 - cleanup
 - cleanup
 - removed allow undo parameter
 - nl
 - test
 - append insert text
 - tests
 - allow undo

Changes: https://git.openjdk.org/jfx/pull/1941/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1941&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366201
  Stats: 218 lines in 16 files changed: 139 ins; 6 del; 73 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