AttributeError: 'QTextBlockUserData' object has no attribute 'syntax_stack' is raised when running "demo.py"

issue with: pyface/ui/qt4/code_editor/pygments_highlighter.py

an AttributeError: 'QTextBlockUserData' object has no attribute 'syntax_stack' is raised when PyQt4 is used as the Qt4 backend. The issue does not appear with PySide.
Checking whether the "syntax_stack" attr exists solves the issue.

```
def highlightBlock(self, qstring):
""" Highlight a block of text.
"""
qstring = unicode(qstring)
prev_data = self.previous_block_data()

if prev_data is not None:
# fix
if hasattr(prev_data, "syntax_stack"):
self._lexer._epd_state_stack = prev_data.syntax_stack
elif hasattr(self._lexer, '_epd_state_stack'):
del self._lexer._epd_state_stack
```

Tested on OSX, with the most recent macports py26-pyqt4