Extend lifetime of decoration to lifetime of Deleted

When a window is closed, a Deleted object will be constructed and the
Window's properties will be copied over to it. The long term plan is to
stop doing that, i.e. keep the Window alive but just flip a few flags to
indicate that it's been closed.

In order to unify decoration management, this change ensures that it's
okay to have decorations live as long as the Deleted.