Genlist: restore old behaviour of item next/prev in group items

Genlist: restore old behaviour of item next/prev in group items

Commit fd82c2521 has changed the behaviour of item_next/prev_get()
in case of genlist with group items (not tree).

The lagacy behaviour was to tread normal items and group items
in a flat way, like if group items was on the same level of
the children normal items.

As the commit already implement bug compatibility, seems to me
the case to restore also this case. Note that this changes only
apply to legacy genlist (I think).

Let me know if this broke something for you...as touching genlist
code is always an "I hope this is right" operation.