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

     1

# SOME DESCRIPTIVE TITLE.

     2

# Copyright (C) is 21st century inquisition

     3

# This file is distributed under the same license as the Beremiz package.

     4

# FIRST AUTHOR <EMAIL@ADDRESS>, YEAR.

     5

#

     6

#, fuzzy

     7

msgid ""

     8

msgstr ""

     9

"Project-Id-Version: Beremiz 1.1\n"

    10

"Report-Msgid-Bugs-To: \n"

    11

"POT-Creation-Date: 2012-09-08 01:21\n"

    12

"PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"

    13

"Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"

    14

"Language-Team: LANGUAGE <LL@li.org>\n"

    15

"Language: \n"

    16

"MIME-Version: 1.0\n"

    17

"Content-Type: text/plain; charset=UTF-8\n"

    18

"Content-Transfer-Encoding: 8bit\n"

    19

    20

# 132431d63d6e4df6a641d5f9e342832b

    21

#: ../../index.rst:4

    22

msgid "Beremiz's documentation"

    23

msgstr ""

    24

    25

# 35991814a0c44578927d8c48b5808eb6

    26

#: ../../index.rst:6

    27

msgid "Contents:"

    28

msgstr ""