gtklistitem: a11y use the child label

a11y software uses the GtkListItemWidget for the label, but it's
private.  Whether the ListItemWidget has one or multiple children,
it can only have a single label.  Set LABELLED_BY to the child
of the ListItemWidget

See: https://gitlab.gnome.org/GNOME/nautilus/-/issues/2460

gtk/gtklistitem.c

@@ -19,6 +19,7 @@
 
 #include "config.h"
 
+#include "gtkaccessible.h"
 #include "gtklistitemprivate.h"
 
 
@@ -320,7 +321,13 @@ gtk_list_item_set_child (GtkListItem *self,
       self->child = child;
 
       if (self->owner)
-        gtk_list_item_widget_add_child (self->owner, child);
+        {
+          gtk_list_item_widget_add_child (self->owner, child);
+          gtk_accessible_update_relation (GTK_ACCESSIBLE (self->owner),
+                                          GTK_ACCESSIBLE_RELATION_LABELLED_BY, child, NULL,
+                                          -1);
+        }
+
     }
 
   g_object_notify_by_pspec (G_OBJECT (self), properties[PROP_ITEM]);

gtk/gtklistitemwidget.c

@@ -689,6 +689,9 @@ gtk_list_item_widget_add_child (GtkListItemWidget *self,
                                 GtkWidget         *child)
 {
   gtk_widget_set_parent (child, GTK_WIDGET (self));
+  gtk_accessible_update_relation (GTK_ACCESSIBLE (self),
+                                  GTK_ACCESSIBLE_RELATION_LABELLED_BY, child, NULL,
+                                  -1);
 }
 
 void