Proper fix for error 'object has no attribute 'getSlave' in EtherCAT extension

traceback:
File "/home/developer/WorkData/PLC/beremiz/beremiz/IDEFrame.py", line 1433, in OnPouSelectedChanged
window.RefreshView()
File "/home/developer/WorkData/PLC/beremiz/beremiz/etherlab/ConfigEditor.py", line 837, in RefreshView
self.RefreshProcessVariables()
File "/home/developer/WorkData/PLC/beremiz/beremiz/etherlab/ConfigEditor.py", line 886, in RefreshProcessVariables
slaves = self.Controler.GetSlaves(**self.CurrentNodesFilter)
File "/home/developer/WorkData/PLC/beremiz/beremiz/etherlab/EthercatMaster.py", line 341, in GetSlaves
for slave in self.Config.getConfig().getSlave():
<type 'exceptions.AttributeError'>:_'lxml.etree._Element'_object_has_no_attribute_'getSlave'

Steps to reproduce problem:

- Add new EtherCAT master
- Add new EthercatNode to the master
- double click on


Revert commit "Dirty fix for error '_object_has_no_attribute_'getSlave' in EtherCAT extension"
[a3ac46366b86a0b237dac93be6b2281ac70b98a8].

The problem was that XML elements (proxy object) in some cases were created using custom XML
classes constructors and lxml.etree.Element() call and live python
patching. This causes that lxml backend doesn't know that custom python class
should be used for these XML elements.
Proxy object can be move/deleted and recreated by lxml
backend at any point in time or this can be done in python by copy/deepcopy operations.
If this happens, then newly created
proxy elements are using default class lxml.etree._Element. And all
custom functionality is lost.

All created XML elements should be always created through corresponding
parser and class lookup callback done by lxml backend.
It's described in more details in lxml documentation:
https://lxml.de/element_classes.html

#!/usr/bin/env python
# -*- coding: utf-8 -*-

# This file is part of Beremiz
#
# Copyright (C) 2007: Laurent BESSARD
# Copyright (C) 2007-2018: Edouard TISSERANT
#
# See COPYING file for copyrights details.

libraries = [
    ('Native', 'NativeLib.NativeLibrary', True),
    ('Python', 'py_ext.PythonLibrary', True),
    ('Etherlab', 'etherlab.EthercatMaster.EtherlabLibrary', False),
    ('SVGUI', 'svgui.SVGUILibrary', False)]

catalog = [
    ('canfestival', _('CANopen support'), _('Map located variables over CANopen'), 'canfestival.canfestival.RootClass'),
    ('bacnet', _('Bacnet support'), _('Map located variables over Bacnet'), 'bacnet.bacnet.RootClass'),
    ('etherlab', _('EtherCAT master'), _('Map located variables over EtherCAT'), 'etherlab.etherlab.RootClass'),
    ('modbus', _('Modbus support'), _('Map located variables over Modbus'), 'modbus.modbus.RootClass'),
    ('c_ext', _('C extension'), _('Add C code accessing located variables synchronously'), 'c_ext.CFile'),
    ('py_ext', _('Python file'), _('Add Python code executed asynchronously'), 'py_ext.PythonFile'),
    ('wxglade_hmi', _('WxGlade GUI'), _('Add a simple WxGlade based GUI.'), 'wxglade_hmi.WxGladeHMI'),
    ('svgui', _('SVGUI'), _('Experimental web based HMI'), 'svgui.SVGUI')]

file_editors = []