Since the last notification redesign, we've been showing the time a
notification was received in the calendar drop-down. However as the
time is in fact added by the NotificationSection, it is actually the
time a notification was added to the list. Usually that difference is
not significant, except when previously received notifications are
restored on startup.
In order to be able to address those cases, we need a time that is
associated with the notification itself, so add a datetime property
that defaults to the current time, but may be set from an optional
parameter as well.
https://bugzilla.gnome.org/show_bug.cgi?id=775799
0569bb18f5