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

<?xml version='1.0' encoding='utf-8'?>
<BeremizRoot URI_location="LOCAL://">
  <TargetType/>
  <Libraries Enable_SVGUI_Library="false"/>
</BeremizRoot>