Set correct accessible relations for GtkMenuButton

Up until now, as the focus was moved to the inner button, it was not possible for
assistive technologies to determine the correct labels and descriptions
because developers could set them only for the parent widget.
Now, the proper relations are added so the labels should be picked up properly.

Fixes #4254

gtk/gtkmenubutton.c

@@ -669,6 +669,10 @@ gtk_menu_button_init (GtkMenuButton *self)
   gtk_widget_set_sensitive (self->button, FALSE);
 
   gtk_widget_add_css_class (GTK_WIDGET (self), "popup");
+
+  gtk_accessible_update_relation (GTK_ACCESSIBLE (self->button), GTK_ACCESSIBLE_RELATION_LABELLED_BY, self, NULL,
+  GTK_ACCESSIBLE_RELATION_DESCRIBED_BY, self, NULL,
+                                  -1);
 }
 
 static GtkBuildableIface *parent_buildable_iface;
@@ -1017,6 +1021,9 @@ gtk_menu_button_set_icon_name (GtkMenuButton *menu_button,
   box = gtk_box_new (GTK_ORIENTATION_HORIZONTAL, 0);
   gtk_widget_set_halign (box, GTK_ALIGN_CENTER);
 
+  // Because we are setting only an icon, let the inner button be labeled by us, so the accessible label can be overridden
+  gtk_accessible_update_relation (GTK_ACCESSIBLE (menu_button->button), GTK_ACCESSIBLE_RELATION_LABELLED_BY, menu_button, NULL, -1);
+
   image_widget = g_object_new (GTK_TYPE_IMAGE,
                                "accessible-role", GTK_ACCESSIBLE_ROLE_PRESENTATION,
                                "icon-name", icon_name,
@@ -1149,6 +1156,9 @@ gtk_menu_button_set_label (GtkMenuButton *menu_button,
   gtk_button_set_child (GTK_BUTTON (menu_button->button), box);
   menu_button->label_widget = label_widget;
 
+  // When the user explicitly set a label assume that it should be announced by assistive technologies as well
+  gtk_accessible_update_relation (GTK_ACCESSIBLE (menu_button->button), GTK_ACCESSIBLE_RELATION_LABELLED_BY, menu_button->label_widget, NULL, -1);
+
   menu_button->image_widget = NULL;
   menu_button->child = NULL;