Adds a token to the text layer
        @type wf_obj: L{Cwf}
        @param wf_obj: the token object

def add_wf(self,wf_obj):
        """
        Adds a token to the text layer
        @type wf_obj: L{Cwf}
        @param wf_obj: the token object
        """
        if self.text_layer is None:
            self.text_layer = Ctext(type=self.type)
            self.root.append(self.text_layer.get_node())
        self.text_layer.add_wf(wf_obj)