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

Remove useless print statement from wxHMI GUI code

It's py3k incompatible.

Rename extension instance 'wxHMI' to 'wxglade_hmi'

It is necessary because of following commit 7f59aa398669 ('WxGlade HMI extension now does
only instanciate wx object who's name match extension's name. If no
object match the name and no code is provided in 'start' user python
code section, then a warning is issued')

Remove backup .bak files