Add event/reminder attributes, reset automatic snooze on automatic dismiss.

See commit messages for details.

These commits move event/reminder properties to attributes instead of changing/appending the APIs: Maemo::Timed::Interface::Event & Maemo::Timed::Voland::Reminder. This has a few advantages:

* Both events and reminders have the set of attributes attached, so event properties will be readily available without need for additional Dbus-based requests (mostly in case of reminders).

* Attributes are appended dynamically, modifying/passing data in attributes does not affect the ABI of timed.