Deleting an unused virtual result class
DDGC::Subscriptions - description and generation of notification events

Passing a category and object instance to generate_events will generate
event table entries for all matching event types.

This is a skeletal implementation with just a couple of event types
described.
Event and Subscription table definitions.

A user's subscriptions dictate which events to notify on.
Subscriptions::Read - per-category timestamps denoting seen notifications