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

[TOPOLOGY]
NetName=None
Nodes=0x01
Node2Present=0x01
Node2Name=SlaveNode
Node2DCFName=Slave_0_0.eds
EDSBaseName=eds