Gets the type.group.label + key spec from an Element in an Overlay.

def get_overlay_spec(o, k, v):
    """
    Gets the type.group.label + key spec from an Element in an Overlay.
    """
    k = wrap_tuple(k)
    return ((type(v).__name__, v.group, v.label) + k if len(o.kdims) else
            (type(v).__name__,) + k)