Also add a README.md to python3-flightgear explaining how to use the
PYTHONPATH environment variable or a .pth file in order to run the
Python scripts in FGMeta, and pointing to the top-level directories
'catalog' and 'i18n'.
- 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.
