Create notifications from full incidence objects
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.