Split up notification module

[Peter Suter]

The Advanced Notification proposal starts with splitting up the trac/notification.py into:

• trac/notification/api.py containing interface definitions and the NotificationSystem.
• trac/notification/mail.py containing any email specific functionality like SmtpEmailSender and SendmailEmailSender.
• trac/notification/compat.py containing the Notify / NotifyEmail base classes. (After #3517 these and TicketNotifyEmail would only remain for backward compatibility and could be deprecated and removed later.)

For backwards compatibility these will also be imported in trac/notification/__init__.py.

I'd also like to extract some useful pieces from NotifyEmail and TicketNotifyEmail into trac/notification/mail.py, so we can reuse them later.

The proposed changes are in log:psuter@advanced-notification-preliminary-refactorings.