Support multiple views

[kylebarron]

code for reference

    # Note: it would be nicer to have only VariableLengthTuple, which would ensure the
    # JS side always receives a list of strings and never a single string, but
    # VariableLengthTuple won't coerce a single object into a tuple of length 1.
    views = t.Union(
        [
            t.Instance(BaseView),
            VariableLengthTuple(t.Instance(BaseView)),
        ],
    ).tag(
        sync=True,
        **ipywidgets.widget_serialization,
    )