Add pmgrowl_notification column to pm_message (do NOT mark as read when closing notifications)

I am sure that this is somehow possible through the schema api to add a notification column to existing tables.

WHat would be better than marking closed notifications as read would be to just mark them as "notified" in an extra table, so then the query can chec for is_new and also pmgrowl_notified too when checking if the message has been notified for.