Block button handler instead of deactivate button

By deactivating the button, the button lost the focus which makes NoModal
selects the wrong sensitive widget and thus the wizard behaves like if it was
launched from the menu instead of from a tab.

issue3353
review1083003
(transplanted from 0d135623d016ce2d8185fcb5790eb22d9d8de254)

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

@@ -184,8 +184,8 @@ class ViewForm(ParserView):
             self.screen.display()
             return
         else:
-            widget.set_sensitive(False)
+            widget.handler_block_by_func(self.button_clicked)
             try:
                 self.screen.button(widget.attrs)
             finally:
-                widget.set_sensitive(True)
+                widget.handler_unblock_by_func(self.button_clicked)