Freestyle Python API improvements - part 1.

* The API syntax of StrokeVertex and StrokeAttribute was updated by means of getter/setter
properties instead of class methods.  Python style modules (including the Parameter Editor
implementation) were updated accordingly.

* Code clean-up was done for a few Python style modules, mostly by removing duplicated
definitions of stroke shaders and fixing indentation.

source/blender/freestyle/intern/python/BPy_Interface0D.cpp

@@ -57,6 +57,8 @@ int Interface0D_Init( PyObject *module )
 	Py_INCREF( &TVertex_Type );
 	PyModule_AddObject(module, "TVertex", (PyObject *)&TVertex_Type);
 
+	StrokeVertex_mathutils_register_callback();
+
 	return 0;
 }