viewDidDisappear may be called when a file is successfully selected.
presentationControllerDidDismiss is only called when the controller is
dismissed manually by the user, e.g. by tapping outside the sheet, or by
dragging it away.

Checking for sheet dismissal is necessary in iOS 15, but not in iOS 17.
In iOS 17, tapping outside the file chooser causes a callback to
documentPickerWasCancelled instead.

This works around a rare issue in the DialogsDemo where the AlertWindow
would fail to display because no peer had focus.

It seems like shared_from_this may not be enabled when a unique_ptr is
assigned to a shared_ptr (although it *should* be enabled when
constructing a new shared_ptr from a unique_ptr). Functions that return
objects that may need to use shared_from_this now return shared_ptr,
just to be safe.

Additionally, in some cases, shared_from_this was being called from
Thread::run after the last reference to the shared object had been
released. We now call shared_from_this during 'open', which will always
run on the message thread while at least once reference to the shared
object is alive.