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 '').