which call gimp_item_get_by_id() and additionally check if the
returned item has the right type, and return NULL if not.
This is both shorter and more readable than
layer = GIMP_LAYER (gimp_item_get_by_id (id));
and additionally makes sure we don't cast e.g. a non-layer with
GIMP_LAYER(), which will give criticals but shouldn't, because the
wrong IDs can come from anywhere and are an input problem and not a
programming error (criticals are for programming errors).
No need of is_id_arg() anymore in pdb/lib.pl. Let's reuse the {id}
value. Also I had to add an additional trick for GimpDisplay which we
will now generate as such in libgimp PDB files, but still need to show
as GimpObject on app/pdb/.
As previously, only the new classes and the PDB generation for a first
step.
Michael Natterer