event: Add API to clone events

This will be useful for updating events without having
to keep track of the event states.

Add the new API, and also a unit test.

src/core/gcal-event.c

@@ -799,6 +799,27 @@ gcal_event_new (GcalCalendar   *calendar,
                          NULL);
 }
 
+/**
+ * gcal_event_new:
+ * @self: a #GcalEvent
+ *
+ * Clones @event into a new #GcalEvent instance. This is useful
+ * for updating events.
+ *
+ * Returns: (transfer full)(nullable): a #GcalEvent
+ */
+GcalEvent*
+gcal_event_new_from_event (GcalEvent *self)
+{
+  g_autoptr (ECalComponent) component = NULL;
+
+  g_return_val_if_fail (GCAL_IS_EVENT (self), NULL);
+
+  component = e_cal_component_clone (self->component);
+
+  return gcal_event_new (self->calendar, component, NULL);
+}
+
 /**
  * gcal_event_get_all_day:
  * @self: a #GcalEvent

src/core/gcal-event.h

@@ -53,6 +53,8 @@ GcalEvent*           gcal_event_new                              (GcalCalendar
                                                                   ECalComponent      *component,
                                                                   GError            **error);
 
+GcalEvent*           gcal_event_new_from_event                   (GcalEvent          *self);
+
 gboolean             gcal_event_get_all_day                      (GcalEvent          *self);
 
 void                 gcal_event_set_all_day                      (GcalEvent          *self,