Don't set image on button if it is still the same

It fixes also a strange behavior of GTK that doesn't display any images if the
icon is changed before it is realized.

issue3191
review1451002
(transplanted from 4bc44691dc89dfd7802c9d84bbb466f98038bb75)

tryton/common/button.py

@@ -17,6 +17,11 @@ class Button(gtk.Button):
         self._set_icon(attrs.get('icon'))
 
     def _set_icon(self, stock):
+        image = self.get_image()
+        if not image and not stock:
+            return
+        elif image and image.get_stock()[0] == stock:
+            return
         if not stock:
             self.set_image(gtk.Image())
             return