Adds a decoration for the word under ``cursor``.

def _add_decoration(self, cursor):
        """
        Adds a decoration for the word under ``cursor``.
        """
        if self.select_word(cursor):
            self.editor.set_mouse_cursor(Qt.PointingHandCursor)
        else:
            self.editor.set_mouse_cursor(Qt.IBeamCursor)