- libgimpbase: change GPParam to transfer all information about the
  GValues we use, in the same way done for GPParamDef. GPParam is now
  different from GimpParam from libgimp, pointers can't be casted any
  longer. The protocol is now completely GimpPDBArgType-free. Remove
  gp_params_destroy() from the public API.

- libgimp: add API to convert between an array of GPParams and
  GimpValueArray, the latter is now the new official API for dealing
  with procedure arguments and return values, GimpParam is cruft (the
  wire now talks with GimpPlugIn more directly than with the members
  of GimpPlugInInfo, which need additional compat conversions).

- libgimp, app: rename gimpgpparamspecs.[ch] to simply
  gimpgpparams.[ch] which is also more accurate because they now
  contain GValue functions too. The code that used to live in
  app/plug-in/plug-in-params.h is now completely in libgimp.

- app: contains no protocol compat code any longer, the only place
  that uses GimpPDBArgType is the PDB query procedure implementation,
  which also needs to change.

- app: change some forgotten int32 run-modes to enums.