Fix Qt CodeEditor pygments monkeypatch conflict with ipython.

Both the Qt CodeEditor and IPython monkeypatch the pygments'
`RegexLexer.get_tokens_unprocessed` in a slightly different manner,
which meant only the one imported later would work correctly.
This is fixed by now using same monkeypatch as ipython, which
is replacing `_epd_state_stack` with `_saved_state_stack`