- 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.