Merge lp://staging/~charlesk/unity-notifications/lp-1470031-ensure-close-notification-signals-are-emitted into lp://staging/unity-notifications

When a notification is closed due to internals (e.g. due to timeout), ensure the NotificationClosed bus signal is emitted.

The regression comes from the new input sanitizing code in NotificationServer::CloseNotification(), which is a public bus method. The sanity test confirms that the notification exists and is owned by the caller.

However this is also used when a Notification is closed internally due to timeout, and we need to handle the case where the Notification has already been removed from the tables but we need to emit nevertheless.

The solution is to break the input sanitizing code bits and the work bits into two separate methods, so that the implementation can do what it needs to without hitting sanity tests that don't apply to it.

(There is another question of why Notification::~Notification() calls CloseNotification() on itself, but that's an issue for a separate refactor...)