core: Add a wrapper object for event organizer

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>

src/core/gcal-event.h

@@ -20,6 +20,7 @@
 #define GCAL_EVENT_H
 
 #include "gcal-calendar.h"
+#include "gcal-event-organizer.h"
 #include "gcal-range.h"
 #include "gcal-recurrence.h"
 
@@ -133,6 +134,10 @@ gchar*               gcal_event_format_date                      (GcalEvent
 gboolean             gcal_event_overlaps                         (GcalEvent          *self,
                                                                   GcalRange          *range);
 
+GSList *             gcal_event_get_attendees                    (GcalEvent          *self);
+
+GcalEventOrganizer * gcal_event_get_organizer                    (GcalEvent          *self);
+
 G_END_DECLS
 
 #endif /* GCAL_EVENT_H */