Dialog._dialogStack holds reference to removed node

`Dialog._dialogStack` items `focus` property is not cleared, so the 0th item may hold a reference to an item removed from DOM i.e. kind of a leak. I suggest one of the following solutions:
* Stop using previous `ds` item and just set previous focus to the current ds item.
* The `focus` in the previous `ds` item is removed on `hide()`.
* The `focus` is held by a weak reference.