Summary
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Removes `allowUndo` parameter from `RichTextArea::replaceText`.
Adds a boolean `undoRedoEnabled` attribute to the `StyledTextModel`.

Note that this is NOT a backward-compatible change, as a part of the incubator iteration.

Problem
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Early user feedback asked for addition of `allowUndo` parameter to `applyStyle()` and `setStyle()` methods in the  `StyledTextModel`, to mirror the parameter in `replaceText()`.

However, selectively enabling or disabling the undo functionality corrupts the internal state.  Instead, to address use cases that require disabled undo, such as programmatically building the document content, a single boolean flag is added to the model.  Disabling undo also clears the undo/redo stack.

Solution
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The solution is to remove the offending parameter and add a flag.

Additionally, similar methods are added to the `RichTextArea` class that forward the calls to the model (or do nothing if the model is null).

Specification
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See the attached diffs.