If we unparent the widget, we should sever a11y relations too.
Otherwise, an a11y implementation might follow them and be surprised
to find a parentless widget (and not in a good way).
Updated tests to not check the relation on an unexpanded expander.
Using GList is a bit lame, and makes the API more complicated to use
than necessary in the common case.
The only real use case for a GList is gtk_widget_add_mnemonic_label(),
and for that we can use the GValue-based API instead.
Fixes: #3343