As far as plug-in API is concerned, at least the calling API, order of arguments
when calling PDB procedures doesn't matter anymore.

Order still matters for creating procedures with standard arguments (for
instance, "run-mode" is first, then image, or file, drawables or whatnot,
depending on the subtype of procedure), but not for calling with libgimp.

Concretely in this commit:

- gimp_pdb_run_procedure_argv() was removed as it's intrinsically order-based.
- gimp_pdb_run_procedure() and gimp_pdb_run_procedure_valist() stay but their
  semantic changes. Instead of an ordered list of (type, value) couple, it's now
  an unordered list of (name, type, value) triplets. This way, you can also
  ignore as many args as you want if you intend to keep them default. For
  instance, say you have a procedure with 20 args and you only want to change
  the last one and keep the 19 first with default values: while you used to have
  to write down ...