Save active tab before we stop engine

I guess it makes sense to introduce a some kind of GlobalSignalDispatcher and to move emitting beforeShutdown and dumpMemoryInfo to this global object from DeclarativeWebUtils, but I'm not sure these two signals justify the split.
We could do it like this or we just add "emit beforeShutdown" signal and emit that from CloseEventFilter.cpp. By doing it this way in future we can add shutdown steps also to webutils.