Destroy the info controller with its content widgets.

createMemento() freed the Controller while _content lived on, and the
content widgets hold it by a raw pointer: MainWidget::showHistory takes
the memento, runs a whole HistoryWidget::showHistory - which reaches
itemVisibilitiesUpdated() and the still-subscribed media list - and only
then destroys the section. The nulling was never a lifetime requirement,
just the "memento already taken" flag showInternal() reads, so make that
a flag and let the controller die with the widget after _content.