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>