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 all 20 args annoyingly, now you can just list the only arg you
  care about.

There are 2 important consequences here:

1. Calling PDB procedures becomes much more semantic, which means scripts with
   PDB calls are simpler (smaller list of arguments) and easier to read (when
   you had 5 int arguments in a row, you couldn't know what they refer to,
   except by always checking the PDB source; now you'll have associated names,
   such as "width", "height" and so on) hence maintain.
2. We will have the ability to add arguments and even order the new arguments in
   middle of existing arguments without breaking compatibility. The only thing
   which will matter will be that default values of new arguments will have to
   behave like when the arg didn't exist. This way, existing scripts will not be
   broken. This will avoid us having to always create variants of PDB procedure
   (like original "file-bla-save", then variant "file-bla-save-2" and so on)
   each time we add arguments.

Note: gimp_pdb_run_procedure_array() was not removed yet because it's currently
used by the PDB. To be followed.