event: implement GInitable

Since the event creation can fail, lets make it a GInitable
so that we can fail during initialization.

https://bugzilla.gnome.org/show_bug.cgi?id=763166

src/gcal-quick-add-popover.c

@@ -465,7 +465,7 @@ edit_or_create_event (GcalQuickAddPopover *self,
   /* Create an ECalComponent from the data above */
   component = build_component_from_details (summary, date_start, date_end);
 
-  event = gcal_event_new (source, component);
+  event = gcal_event_new (source, component, NULL);
   gcal_event_set_all_day (event, all_day);
   gcal_event_set_timezone (event, tz);