app: make undo compression in the text tool work again
- in GimpText, make "text" and "markup" mutually exclusive, so that whenever one is set to non-NULL, the other is cleared automatically. - add gimp_text_buffer_has_markup() which returns TRUE if any char in the buffer is tagged. - in the text tool, only set "markup" on the text proxy if there is actually markup in the buffer, and set "text" otherwise. This way we don't push "text" *and* "markup" undos on each keystroke, and undo compression works the way it did before.
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