GtkStackSidebar: Explicitly mark the sidebar items as labeled by the labels

That improves the reading of the GtkStackSidebar widget considerably.
Previously, Orca would not read the items at all, now it does.

gtk/gtkstacksidebar.c

@@ -220,6 +220,12 @@ add_child (guint            position,
   row = gtk_list_box_row_new ();
   gtk_list_box_row_set_child (GTK_LIST_BOX_ROW (row), item);
 
+  gtk_accessible_update_relation (GTK_ACCESSIBLE (row),
+                                  GTK_ACCESSIBLE_RELATION_LABELLED_BY,
+                                  item,
+                                  NULL,
+                                  -1);
+
   page = g_list_model_get_item (G_LIST_MODEL (self->pages), position);
   update_row (self, page, row);