A widget view should call its .remove() method to destroy itself.
(though I don't know if that actually happens when reexecuting a cell)
@takluyver, did you see our report of a problem with the callback-based view creation? Now, suddenly, order is not guaranteed for view creation, which violates some assumptions that were made in the box classes, for example
(i.e., if I have a list of children, now the order of their creation is not guaranteed, while before it was)