Make the presentation of the stack sidebar listbox nicer for A11Y
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 (closed).