Load images for EtherCAT extension

Fixes following backtrace:

Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk3/wx/_core.py", line 16765, in <lambda>
lambda event: event.callable(*event.args, **event.kw) )
File "/home/developer/WorkData/PLC/beremiz/beremiz/BeremizIDE.py", line 1074, in AddConfNode
self._Refresh(TITLE, FILEMENU, PROJECTTREE)
File "/home/developer/WorkData/PLC/beremiz/beremiz/IDEFrame.py", line 917, in _Refresh
self.RefreshFunctions[element]()
File "/home/developer/WorkData/PLC/beremiz/beremiz/IDEFrame.py", line 1526, in RefreshProjectTree
self.GenerateProjectTreeBranch(root, infos)
File "/home/developer/WorkData/PLC/beremiz/beremiz/IDEFrame.py", line 1565, in GenerateProjectTreeBranch
self.GenerateProjectTreeBranch(item, values)
File "/home/developer/WorkData/PLC/beremiz/beremiz/IDEFrame.py", line 1554, in GenerateProjectTreeBranch
self.TreeImageDict[icon_name] = self.TreeImageList.Add(GetBitmap(icon_name))
File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk3/wx/_gdi.py", line 6735, in Add
return _gdi_.ImageList_Add(*args, **kwargs)
ValueError: invalid null reference in method 'ImageList_Add', expected argument 2 of type 'wxBitmap const &

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# This file is part of Beremiz.
# See COPYING file for copyrights details.

from __future__ import absolute_import
from plcopen.XSLTModelQuery import XSLTModelQuery


class InstancesPathCollector(XSLTModelQuery):
    """ object for collecting instances path list"""
    def __init__(self, controller):
        self.Instances = []
        XSLTModelQuery.__init__(self,
                                controller,
                                "instances_path.xslt",
                                [("AddInstance", self.AddInstance)])

    def AddInstance(self, context, *args):
        self.Instances.append(args[0][0])

    def Collect(self, root, name, debug):
        self._process_xslt(root, debug, instance_type=name)
        res = self.Instances
        self.Instances = []
        return res