Adds a timex entry to the time layer
        @type time_obj: L{Ctime}
        @param time_obj: time time object

def add_timex(self, time_obj):
        """
        Adds a timex entry to the time layer
        @type time_obj: L{Ctime}
        @param time_obj: time time object
        """
        if self.timex_layer is None:
            self.timex_layer = CtimeExpressions()
            self.root.append(self.timex_layer.get_node())
        self.timex_layer.add_timex(time_obj)