As of now, when the stack sidebar listbox is focused, Orca reports the default name of GtkListBox to A11Y. That's a technical detail, so this MR replaces it at least with something humanly readable. I am not sure whether we want to allow configuring this label from outside or whether we want to convince A11Y to not read a label at all, but this first version should at least make someone notice this issue. This fixes #4577.