Convert and use actuators with no target object set.

Certain actuators (Add Object/Replace Mesh/Scene/Track To) used to always require a target, or they would not influence the scene.  If the actuator target is always set from Python, this could be annoying.

source/gameengine/Ketsji/KX_SCA_AddObjectActuator.cpp

@@ -263,6 +263,9 @@ PyObject* KX_SCA_AddObjectActuator::PyGetObject(PyObject* self,
 												PyObject* args,
 												PyObject* kwds)
 {
+	if (!m_OriginalObject)
+		Py_Return;
+
 	return PyString_FromString(m_OriginalObject->GetName());
 }