Emitted when @jar changes. If a cookie has been added,
@new_cookie will contain the newly-added cookie and
@old_cookie will be %NULL. If a cookie has been deleted,
@old_cookie will contain the to-be-deleted cookie and
@new_cookie will be %NULL. If a cookie has been changed,
@old_cookie will contain its old value, and @new_cookie its
new value.
Emitted when @jar changes. If a cookie has been added, @new_cookie will contain the newly-added cookie and @old_cookie will be %NULL. If a cookie has been deleted, @old_cookie will contain the to-be-deleted cookie and @new_cookie will be %NULL. If a cookie has been changed, @old_cookie will contain its old value, and @new_cookie its new value.