Adds a markable to the markable layer
        @type markable_obj: L{Cmarkable}
        @param markable_obj: the markable object

def add_markable(self,markable_obj):
        """
        Adds a markable to the markable layer
        @type markable_obj: L{Cmarkable}
        @param markable_obj: the markable object
        """
        if self.markable_layer is None:
            self.markable_layer = Cmarkables(type=self.type)
            self.root.append(self.markable_layer.get_node())
        self.markable_layer.add_markable(markable_obj)