Add a wrapper for the ECalComponentOrganizer. An instance of this type is kept as part of the GcalEvent structure. The GcalEventOrganizer is just a "proxy" to the ECalComponentAttendee and its purpose is to unclutter the GcalEvent interface and provide a bindable base for a UI widget. Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/671>