Additional fix for rewrite ForceVariableDialog commit (9076249f)

fix 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/controls/DebugVariablePanel/DebugVariableViewer.py", line 232, in HandleButton
button.ProcessCallback()
File "/home/developer/WorkData/PLC/beremiz/beremiz/controls/DebugVariablePanel/GraphButton.py", line 156, in ProcessCallback
self.Callback()
File "/home/developer/WorkData/PLC/beremiz/beremiz/controls/DebugVariablePanel/DebugVariableViewer.py", line 350, in OnForceButton
self.ForceValue(self.ItemsDict.values()[0])
File "/home/developer/WorkData/PLC/beremiz/beremiz/controls/DebugVariablePanel/DebugVariableViewer.py", line 428, in ForceValue
dialog.GetValue())
File "/home/developer/WorkData/PLC/beremiz/beremiz/dialogs/ForceVariableDialog.py", line 232, in GetValue
return GetTypeValue[self.IEC_Type](wx.TextEntryDialog.GetValue(self))
TypeError: unbound method GetValue() must be called with TextEntryDialog instance as first argument (got ForceVariableDialog instance instead)

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#EDE (Engineering-Data-Exchange) - generated by Beremiz BACnet plugin;;;;;;;;;;;;;;;

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PROJECT_NAME;%(Project Name)s;;;;;;;;;;;;;;

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VERSION_OF_REFERENCEFILE;%(EDE file version)d;;;;;;;;;;;;;;

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TIMESTAMP_OF_LAST_CHANGE;%(Current Time)s;;;;;;;;;;;;;;

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AUTHOR_OF_LAST_CHANGE;%(Project Author)s;;;;;;;;;;;;;;

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VERSION_OF_LAYOUT;2.1;;;;;;;;;;;;;;

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#mandatory;mandatory;mandatory;mandatory;mandatory;optional;optional;optional;optional;optional;optional;optional;optional;optional;optional;optional

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#keyname;device obj.-instance;object-name;object-type;object-instance;description;present-value-default;min-present-value;max-present-value;settable;supports COV;hi-limit;low-limit;state-text-reference;unit-code;vendor-specific-address