The idea is that we already have a GimpProcedure object in libgimp
which has name, help, blurb, arguments, return values and everything,
so we really don't need a parallel API to query PDB procedures for
their properties.

- make run() a virtual function of GimpProcedure
- move GIMP_PDB_ERROR to GimpPDB
- GimpPDBProcedure is a trivial subblass which populates
  GimpProcedure's members by querying the PDB.
- make "plug-in", "procedure-type" and "name" construct-only
  properties of GimpProcedure.

This is all work in progress.