GtkContainer: Add method to get the GtkWidgetPath for a child.

This is now used throughout in order to have the full path for a given widget,
including intermediate named regions, the default implementation just returns
the GtkContainer's path copy, no intermediate regions between.

gtk/gtkwidgetpath.c

@@ -128,6 +128,21 @@ gtk_widget_path_prepend_type (GtkWidgetPath *path,
   g_return_val_if_fail (path != NULL, 0);
   g_return_val_if_fail (g_type_is_a (type, GTK_TYPE_WIDGET), 0);
 
+  new.type = type;
+  g_array_prepend_val (path->elems, new);
+
+  return 0;
+}
+
+guint
+gtk_widget_path_append_type (GtkWidgetPath *path,
+                             GType          type)
+{
+  GtkPathElement new = { 0 };
+
+  g_return_val_if_fail (path != NULL, 0);
+  g_return_val_if_fail (g_type_is_a (type, GTK_TYPE_WIDGET), 0);
+
   new.type = type;
   g_array_append_val (path->elems, new);