Fix wxHMI example after upgrading wxGlade

Newer wxGlade generates code to initialize GridSizer's with empty
elements.

...
grid_sizer_1.Add(self.window_1, 1, wx.ALIGN_CENTER, 0)
sizer_2.Add((0, 0), 0, 0, 0)
sizer_2.Add((0, 0), 0, 0, 0)
sizer_2.Add((0, 0), 0, 0, 0)
sizer_2.Add((0, 0), 0, 0, 0)
...

That causes following traceback, if new buttons are added
to already full sizer.

PLCobject : Traceback (most recent call last):

File "./Beremiz_service.py", line 389, in default_evaluator
res = (tocall(*args, **kwargs), None)

File "/tmp/tmpQS8ct2/runtime_0.py", line 540, in _runtime_0_start
wx.MessageBox(_("Please stop PLC to close"))

File "/tmp/tmpQS8ct2/runtime_0.py", line 504, in Init
lambda axis:( MakeButtonFunc(self, sizer, axis+"axisMinus"),

File "/tmp/tmpQS8ct2/runtime_0.py", line 502, in <lambda>
lambda btname: MakeButtonFunc(self, sizer, btname), ActionButtons)

File "/tmp/tmpQS8ct2/runtime_0.py", line 461, in MakeButtonFunc
print sizer, btname

File "/usr/lib/python2.7/dist-packages/wx-3.0-gtk3/wx/_core.py", line 14453, in Add
return _core_.Sizer_Add(*args, **kwargs)

PyAssertionError: C++ assertion "Assert failure" failed at
../src/common/sizer.cpp(1401) in DoInsert(): too many items (11 > 2*5)
in grid sizer (maybe you should omit the number of either rows or
columns?)


Tested with wxGlade version 0.8.3
Closes #41

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<?xml version="1.0" encoding="ISO-8859-1" ?>

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<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"

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            xmlns:xhtml="http://www.w3.org/1999/xhtml">

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  <xsd:element name="Python">

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    <xsd:complexType>

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      <xsd:annotation>

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        <xsd:documentation>Formatted text according to parts of XHTML 1.1</xsd:documentation>

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      </xsd:annotation>

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      <xsd:sequence>

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        <xsd:any namespace="http://www.w3.org/1999/xhtml" processContents="lax"/>

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      </xsd:sequence>

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    </xsd:complexType>

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  </xsd:element>

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</xsd:schema>