… a GimpProcedureConfig for arguments.

This also factorizes the code to load metadata. By default, a GimpLoadProcedure
will try and load metadata from a file (if Exiv2 knows the format). The run()
function will be allowed to edit the GimpMetadata object but also the load flags
before it is actually attached to the image, allowing plug-ins to have custom
metadata handling code when needed.