Ability to specify the comment index when updating a ticket

The 'index' is not something returned by ticket.changeLog() to allow you to read it, nor is it acutally part of the database schema for ticket_change. Using the data we have, what about a new method that looks something like this:

int ticket.updateChange(int ticket, datetime time, string field, string newvalue)

It would update the unique record (ticket,time,field) with content newvalue.

If done like this, the method should also be accompanied by a similar ticket.deleteChange() method that deletes the record (using same identification mechanism).

Both these methods should require TICKET_ADMIN permission for the resource.