flattenlistmodel: Give access to child models

Add an api to retrieve the model containing a given
item in a flatten listmodel. This is useful when the
individual items in the list don't have backpointers.

docs/reference/gtk/gtk4-sections.txt

@@ -1316,6 +1316,7 @@ GtkFlattenListModel
 gtk_flatten_list_model_new
 gtk_flatten_list_model_set_model
 gtk_flatten_list_model_get_model
+gtk_flatten_list_model_get_model_for_item
 <SUBSECTION Standard>
 GTK_FLATTEN_LIST_MODEL
 GTK_IS_FLATTEN_LIST_MODEL

gtk/gtkflattenlistmodel.c

@@ -539,3 +539,28 @@ gtk_flatten_list_model_get_model (GtkFlattenListModel *self)
 
   return self->model;
 }
+
+/**
+ * gtk_flatten_list_model_get_model_for_item:
+ * @self: a #GtkFlattenListModel
+ * @position: a position
+ *
+ * Returns the model containing the item at the given position.
+ *
+ * Returns: (transfer none): the model containing the item at @position
+ */
+GListModel *
+gtk_flatten_list_model_get_model_for_item (GtkFlattenListModel *self,
+                                           guint                position)
+{
+  FlattenNode *node;
+
+  if (!self->items)
+    return NULL;
+
+  node = gtk_flatten_list_model_get_nth (self->items, position, NULL);
+  if (node == NULL)
+    return NULL;
+
+  return node->model;
+}

gtk/gtkflattenlistmodel.h

@@ -45,6 +45,10 @@ void                    gtk_flatten_list_model_set_model        (GtkFlattenListM
 GDK_AVAILABLE_IN_ALL
 GListModel *            gtk_flatten_list_model_get_model        (GtkFlattenListModel    *self);
 
+GDK_AVAILABLE_IN_ALL
+GListModel *            gtk_flatten_list_model_get_model_for_item (GtkFlattenListModel    *self,
+                                                                   guint                   position);
+
 G_END_DECLS
 
 #endif /* __GTK_FLATTEN_LIST_MODEL_H__ */