ListBox: fix possible unexpected acquisition

The use case is a listbox method being called for a document A
and returning objects that:
- aren't children of A
- but wrapped with `.__of__(A)`

By using `aq_self` instead of `aq_base`, the listbox could still
acquire from the real parent of the returned object and wrongly access
an attribute directly (e.g. comment) instead of using an appropriate
method (if there's no `comment` attribute, getComment returns '').

See merge request !1820