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

/*

Template C code used to produce target Ethercat C code.

Copyright (C) 2011-2014: Laurent BESSARD, Edouard TISSERANT

Distributed under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

See COPYING file for copyrights details.

*/

#include "iec_types_all.h"

#define FREE 0
#define ACQUIRED 1
#define ANSWERED 2

long SDOLock = FREE;
extern long AtomicCompareExchange(long* atomicvar,long compared, long exchange);

int AcquireSDOLock() {
	return AtomicCompareExchange(&SDOLock, FREE, ACQUIRED) == FREE;
}

void SDOAnswered() {
	AtomicCompareExchange(&SDOLock, ACQUIRED, ANSWERED);
}

int HasAnswer() {
	return SDOLock == ANSWERED;
}

void ReleaseSDOLock() {
	AtomicCompareExchange(&SDOLock, ANSWERED, FREE);
}

int __init_etherlab_ext()
{
    SDOLock = FREE;
    return 0;
}

void __cleanup_etherlab_ext()
{
}

void __retrieve_etherlab_ext()
{
}

void __publish_etherlab_ext()
{
}