gtkhidingbox: add getter for overflow children

Useful to know which children cannot be shown, so clients
can manage themselves those.
A use case is GtkPathBar for the popup overflow menu.

gtk/gtkhidingbox.c

@@ -524,3 +524,21 @@ gtk_hiding_box_get_inverted (GtkHidingBox *box)
 
   return priv->inverted;
 }
+
+GList *
+gtk_hiding_box_get_overflow_children (GtkHidingBox *box)
+{
+  GtkHidingBoxPrivate *priv ;
+  GList *result = NULL;
+  GList *l;
+
+  g_return_val_if_fail (GTK_IS_HIDING_BOX (box), 0);
+
+  priv = gtk_hiding_box_get_instance_private (box);
+
+  for (l = priv->children; l != NULL; l = l->next)
+    if (gtk_widget_is_visible (l->data) && !gtk_widget_get_child_visible (l->data))
+      result = g_list_append (result, l->data);
+
+  return result;
+}

gtk/gtkhidingboxprivate.h

@@ -70,6 +70,8 @@ void              gtk_hiding_box_set_inverted            (GtkHidingBox      *box
 GDK_AVAILABLE_IN_3_20
 gboolean          gtk_hiding_box_get_inverted            (GtkHidingBox      *box);
 
+GDK_AVAILABLE_IN_3_20
+GList             *gtk_hiding_box_get_overflow_children  (GtkHidingBox      *box);
 G_END_DECLS
 
 #endif /* GTK_HIDING_BOX_PRIVATE_H_ */