SelectMultiple widget

Adds a minimal multiple select widget, with `_MultipleSelection` as baseline for additional multiple select widgets: i.e. `ToggleButtonsMultiple`.

Unlike the suggestion of #5327 to have `multiple=True`, this adopts a whole separate widget due to  downstream incompatibilities: some of the `_Selection` subclasses cannot accept multiple values (Radios). Perhaps adding a `multiple` traitlet while allowing implementations to opt-out would make more sense.