Add property checker/metadata system and C++ helpers

[garyo]

• include/ofx-props.yml is the property reference/metadata
• scripts/gen-props.py is the checker and source generator

All props are included in the YML definition, and the script generates C++ files with all the property metadata and suite assignments. The C++ files aren't yet checked in, but you can build them easily: python scripts/gen-props.py -v and then look in include/gen_*.hxx.