So a value array can now we created like this:

array = gimp_value_array_new_from_types (&error_msg,
                                         G_TYPE_STRING, "foo",
                                         G_TYPE_INT,    23,
                                         G_TYPE_NONE);

Change PDB generation to use this, which makes for much nicer code in
the libgimp wrappers, and only set arrays separately instead of all
values.