Improve hack of glitch in position of widgets in notebook

By calling display when switching tabs, it loose the value of focused widget
because value is reset before the focus-out is proceed.

issue2571
review353001
(transplanted from 9c863c7e1626ca96677d04de19ab59957f5d7253)

tryton/gui/window/view_form/view/form.py

@@ -30,6 +30,7 @@ class ViewForm(ParserView):
         for notebook in notebooks:
             def switch(notebook, page, page_num):
                 if page_num not in display_done.setdefault(notebook, []):
+                    notebook.grab_focus()
                     display_done[notebook].append(page_num)
                     self.display()
             notebook.connect('switch-page', switch)