fix deprecation warnings about accessing properties m_x, m_y of wxKeyEvent

/home/beremiz/beremiz/controls/PouInstanceVariablesPanel.py:354: wxPyDeprecationWarning: Accessing deprecated property.
event.m_x = size.width / 2

import os
from POULibrary import POULibrary

class NativeLibrary(POULibrary):
    def GetLibraryPath(self):
        return os.path.join(os.path.split(__file__)[0], "NativeLib.xml")