1.1.3dev: Include ''format'' in default notification subscriptions. (refs #11875)
Modify the `INotificationSubscriber` interface so `default_subscriptions()` includes a ''format'' in the tuples.
So far the callers just added a fixed format. Including it here seems more consistent and flexible (for configurable defaults).

git-svn-id: http://trac.edgewall.org/intertrac/log:/trunk@13552 af82e41b-90c4-0310-8c96-b1721e28e2e2