app, libgimp, pdb, plug-ins: batch CLI options now interpreter aware.

The CLI options now know which procedures are batch procedures or not.
First it means that it won't just randomly try any procedure name one
may pass and will properly output an error if you pass a non-existing
interpreter procedure.

Secondly, there is no default interpreter anymore (unless only one
interpreter exists). If you don't set an interpreter procedure with
--batch-interpreter or if you pass a wrong one, it will output the list
of available batch procedure, thus helping you understanding how to use
the --batch option.

plug-ins/python/python-eval.py

@@ -42,7 +42,7 @@ class PythonEval (Gimp.PlugIn):
         return ['python-fu-eval']
 
     def do_create_procedure(self, name):
-        procedure = Gimp.BatchProcedure.new(self, name,
+        procedure = Gimp.BatchProcedure.new(self, name, "Python 3",
                                             Gimp.PDBProcType.PLUGIN,
                                             code_eval, None)
         procedure.set_documentation ("Evaluate Python code",