@staticmethod
def addable_cells(obj):
    """
        List object addable cells
        TESTS ::

            sage: from sage.combinat.skew_tableau import SkewTableau
            sage: from sage_widget_adapters.combinat.skew_tableau_grid_view_adapter import SkewTableauGridViewAdapter
            sage: st = SkewTableau([[None, None, None, 1], [None, None, 2], [None, 1], [4]])
            sage: SkewTableauGridViewAdapter.addable_cells(st)
            [(0, 2), (1, 1), (2, 0), (0, 4), (1, 3), (2, 2), (3, 1), (4, 0)]
        """
    return obj.shape().inner().corners() + obj.shape().outer().outside_corners(
        )
