ERP5OOo: don't fail exporting documents with control character

if for some reason an ERP5 document has some control characters in title of
description and is exported in ODS/ODT, the export will fail with an etree
error like this:

    Exception:
      Module Products.CMFActivity.ActivityTool, line 356, in __call__
          result = method(*self.args, **self.kw)
      Module Products.ERP5Type.patches.PythonScript, line 179, in __call__
          return self._orig_bindAndExec(args, kw, None)
      Module Shared.DC.Scripts.Bindings, line 359, in _bindAndExec
          return self._exec(bound_data, args, kw)
      Module Products.PythonScripts.PythonScript, line 344, in _exec
          result = f(*args, **kw)
      Module script, line 15, in Base_renderSimpleView
      - <PythonScript at /erp5/Base_renderSimpleView used for /erp5/sale_packing_list_module>
      - Line 15
          report_data = getattr(context, deferred_style_dialog_method)(**params)
      Module AccessControl.ZopeGuards, line 369, in guarded_apply
          return builtin_guarded_apply(func, args, kws)
      Module AccessControl.ZopeGuards, line 391, in builtin_guarded_apply
          return func(*arglist, **argdict)
      Module Products.ERP5Form.Form, line 705, in __call__
          return pt.pt_render(extra_context=extra_context)
      Module Products.ERP5OOo.OOoTemplate, line 484, in pt_render
          extra_context, request)
      Module Products.ERP5OOo.OOoTemplate, line 422, in renderIncludes
          xml_doc = etree.XML(text)
      Module lxml.etree, line 3192, in lxml.etree.XML (src/lxml/lxml.etree.c:78763)
      Module lxml.etree, line 1848, in lxml.etree._parseMemoryDocument (src/lxml/lxml.etree.c:118341)
      Module lxml.etree, line 1736, in lxml.etree._parseDoc (src/lxml/lxml.etree.c:117021)
      Module lxml.etree, line 1102, in lxml.etree._BaseParser._parseDoc (src/lxml/lxml.etree.c:111265)
      Module lxml.etree, line 595, in lxml.etree._ParserContext._handleParseResultDoc (src/lxml/lxml.etree.c:105109)
      Module lxml.etree, line 706, in lxml.etree._handleParseResult (src/lxml/lxml.etree.c:106817)
      Module lxml.etree, line 635, in lxml.etree._raiseParseError (src/lxml/lxml.etree.c:105671)
    XMLSyntaxError: PCDATA invalid Char value 20, line 228761, column 21 (line 228761)

This is because XML does not accept the full range of characters, there are a
few characters that are not allowed.

To prevent these errors, we replace these invalid characters by an error
character (�) before manipulating the XML with lxml.