Set missing transient to calendar popup

The transient is set just before the popup is shown as the calendar widget
could have changed of toplevel in the meanwhile.

issue4769
review13171002

tryton/common/datetime_.py

@@ -106,6 +106,7 @@ class Date(gtk.Entry):
             self.__calendar.select_month(
                 self.__date.month - 1, self.__date.year)
             self.__calendar.select_day(self.__date.day)
+        self.__cal_popup.set_transient_for(self.get_toplevel())
         popup_position(self, self.__cal_popup)
         popup_show(self.__cal_popup)