Fix bookmarks updating between multiple open windows

With two instances of Okular open, a change made to a bookmark in one won't show up in the other.
This is because Okular::BookmarkManager ignores KBookmarkManager::changed("") firing (where an empty address indicates any bookmark may have changed). In addition, KBookmarks held in BookmarkList by the second window then become stale, so any subsequent changes will appear in the list but won't be saved to disk.

This is fixed by handling changed("") properly.

Thanks.