recurrence: add GcalRecurrenceModType

Instead of reusing the ECalObjModType and mixing the
prefixes all around, do this small cleanup and add a
gcal-prefixed enum.

src/gcal-edit-dialog.c

@@ -914,7 +914,7 @@ remove_button_clicked (GtkButton *button,
 
   gcal_event_remove_alarm (event, trigger_minutes);
 
-  gcal_manager_update_event (self->manager, event, E_CAL_OBJ_MOD_THIS);
+  gcal_manager_update_event (self->manager, event, GCAL_RECURRENCE_MOD_THIS_ONLY);
 
   gtk_widget_destroy (row);