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

<?xml version="1.0"?>
<purchaseOrder orderDate="1999-10-20">
  <shipTo country="US">
    <name>Alice Smith</name>
    <street>123 Maple Street</street>
    <city>Mill Valley</city>
    <state>CA</state>
    <zip>90952</zip>
  </shipTo>
  <billTo country="US">
    <name>Robert Smith</name>
    <street>8 Oak Avenue</street>
    <city>Old Town</city>
    <state>PA</state>
    <zip>95819</zip>
  </billTo>
  <comment>Hurry, my lawn is going wild!</comment>
  <items>
    <item partNum="872-AA">
      <productName>Lawnmower</productName>
      <quantity>1</quantity>
      <USPrice>148.95</USPrice>
      <comment>Confirm this is electric</comment>
    </item>
    <item partNum="926-AA">
      <productName>Baby Monitor</productName>
      <quantity>1</quantity>
      <USPrice>39.98</USPrice>
      <shipDate>1999-05-21</shipDate>
    </item>
  </items>
</purchaseOrder>