- Use the new flightgear.meta.strutils.simplify() function.
- Automatically add a comment to the generated file to indicate that it
was automatically generated and shouldn't be manually modified, etc.
- Improve error handling: abort with a clear error message if any of the
'id', 'name' and 'desc' children of a 'scenario' element is
problematic (empty or missing, for instance).
- Rename make_xml_leaf() to makeXmlLeaf() + other minor renaming.
