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

--- a/tests/wxHMI/wxglade_hmi@wxglade_hmi/pyfile.xml	Mon Aug 13 18:21:10 2018 +0300
+++ b/tests/wxHMI/wxglade_hmi@wxglade_hmi/pyfile.xml	Mon Aug 13 18:29:07 2018 +0300
@@ -91,13 +91,21 @@
 
 #Class_wxglade_hmi.UpdatePositions = UpdatePositions
 
+def CleanSizer(sizer):
+    sizer_len = sizer.GetItemCount()
+    if sizer_len > 0:
+        for i in reversed(range(sizer_len)):
+            sizer.Remove(i)
+
 initorig = Class_wxglade_hmi.__init__
 def Init(self,*args,**kargs):
     initorig(self,*args,**kargs)
     sizer = self.GetSizer().GetItem(1).GetSizer().GetItem(0).GetSizer()
+    CleanSizer(sizer)
     self.main_buttons = map(
         lambda btname: MakeButtonFunc(self, sizer, btname), PwrButtons)
     sizer = self.GetSizer().GetItem(1).GetSizer().GetItem(1).GetSizer()
+    CleanSizer(sizer)
     self.main_buttons = map(
         lambda btname: MakeButtonFunc(self, sizer, btname), ActionButtons)
     self.axis_buttons = map(