Selection containers (Tab, accordion) should allow a selected index of None for consistency

[jasongrout]

This would make it consistent with the selection widgets.

[pbugnion]

The selection widgets explicitly disallow out of range indices.

Do we want to do that for the selection containers as well? It breaks backwards compatibility (I'm not sure how much user code relies on being able to set / get -1 for selected_index), but it would be more consistent and ensures that there's only one way to set the container as having nothing selected.

[jasongrout]

Fixed by #1495.

[SylvainCorlay]

@jasongrout I wonder if the default should not also be None. At the moment, for some reason, the default is 0.

[jasongrout]

I think that would be confusing in most cases. Let's carry on discussion on #1822, though.