If package.xml metadata file exists, it is scanned for FreeCAD version
compatibility before the Addon is loaded. If the Addon specifies that it is
explicitly not compatible with the current version of FreeCAD, the Addon is
not loaded.
Rather than trying to use a generic Py::Object and assigning properties
to it, use a Py::Dict object for simple data structures like Url,
Maintainer, and Author.
Update metadata standard to include subdirectory
