Add support for "sealed containers"

[davep]

Prompted by this conversation, and the need to do this in a couple or so widgets, it would make sense to have a way of having a container of widgets such that the contents are projected from external changes to the DOM. A classic example being the ListView where someone could mount something that isn't a ListItem, or RadioSet where someone could do the same, or remove a RadioButton or two. This is generally going to be an issue for any sort of compound widget.

Which such widgets can advertise that changing up their content like this could lead to unintended consequences, it would be ideal if there was a way of protecting the widget's content.

[willmcgugan]

On reflection, I think this might prevent customization. It is often inadvisable if you didn't write the widget yourself, but I don't want to restrict it.

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