Replace uses of ticket.values with Ticket item accessors

[Ryan J Ollos]

The ticket class implements __getitem__ and __setitem__ to set and get from the values attribute, but many places in the code still work with the values attribute directly. For example, ticket.values['reporter'] has been used since [1831] but it appears it can be replaced with ticket['reporter'].

Directly setting to values could even be problematic due to the logic in __setitem__: tags/trac-1.1.6/trac/ticket/model.py#L186​.

I considered trying to make values a protected attribute, but it is used to serialize the ticket properties:

• tags/trac-1.1.6/trac/ticket/notification.py@:636#L633​.
• tags/trac-1.1.6/trac/ticket/notification.py@:678#L658​.