Move more event fields to EditEngine

Summary:
Ref T9275. This moves description, icon, and cancel/uncancel to EditEngine.

It removes TYPE_SEQUENCE_INDEX and TYPE_INSTANCE_OF_EVENT. These are currently never generated and I do not expect to genereate them (instead, these changes happen automatically when you edit a stub).

Test Plan: Edited an event with normal and pro edit forms.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9275

Differential Revision: https://secure.phabricator.com/D16264