This goes with our planned change of not making GimpProcedure arguments order
relevant anymore regarding the PDB API. In particular, it means we don't want to
use GimpValueArray for various procedure arguments API, but directly
GimpProcedureConfig objects.

This change will allow to add or reorder arguments in the future, so that we
won't have to create new PDB procedures when adding new arguments, while still
keeping PDB API stability.