This allows us to use KNotifications to its full potential, using its
various text fields, icons, etc, and to update notifications as the
incidence changes.

Additionally, this adds a third action to the notification, which so
far can be one of the following:
- Open a URL found in the event. This is useful for the common use-case of
joining an online meeting.
- Open a map at the location of the event.

We also no longer force the notification to critical urgency in order to
avoid it auto-closing, but use the persistence flag for that. This avoids
us interfering with notification inhibiting, and makes this work on systems
that don't imply persistence from critical urgency like Plasma does.

One major feature is still missing now, opening the incidence in a
calendaring app. That's supposed to be bound to the notification default
action, but needs a D-Bus interface on the calendaring app side first.