Fix destruction order of alive and deleted windows

It's needed to work around the cleanup logic of decorations.

Currently, decorations have a valid QObject parent and they're managed
using std::shared_ptr. That's not a perfect combination, but changing it
is also going to be an involved task because the QObject parent is used
to look up the Window.

In long term, it won't matter since we want to get rid of Deleted.

For what it's worth, it restores the order in which Deleted and normal
windows used to be destroyed prior to
995d509e.