Compute a dict of {(row,column): (key, value)} elements from the
        current set of items and specified number of columns.

def grid_items(self):
        """
        Compute a dict of {(row,column): (key, value)} elements from the
        current set of items and specified number of columns.
        """
        if list(self.keys()) == []:  return {}
        cols = self._max_cols
        return {(idx // cols, idx % cols): (key, item)
                for idx, (key, item) in enumerate(self.data.items())}