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)

IEC 61131-3
===========

    IEC-61131 is a normative document provided by the standards organization IEC
    (International Electrotechnical Commission) and describing a standard for
    implementing programmable controllers.


    The part 3 of this document (commonly named IEC 61131-3) specifies syntax and 
    semantics for programming language for programmable controllers. Beremiz
    implements all the languages described in this document. 


    http://www.iec.eu
        
PLCopen TC6
===========
   
    PLCopen is a vendor- and product-independent worldwide association defining
    international standards for various topics related to control programming.
    For this purpose, PLCopen has 6 technical committees.
   
   
    The goal of the sixth committee (TC6) is to define a standard file format, 
    based on XML, for exchanging programmables controllers programmed using 
    IEC 61131-3 languages. Beremiz uses this file format for saving the PLC
    programs of projects.
   
   
    http://www.plcopen.org