genlist: Fix insert after with a tree

This fixes a case where inserting item "C" after item "B" in this
tree would go wrong:

A
B
- 1
- 2

Before this patch, 1 and 2 lose their parent:

A
B
C
- 1
- 2

After this patch, 1 and 2 retain their parent:

A
B
- 1
- 2
C

Insert before worked by luck, no need to fix it.
Note that this patch may require the next one to actually
work (ensuring expanded state flag).

NOTE: This is a behaviour break!

src/lib/elementary/elm_genlist.c

@@ -6356,6 +6356,12 @@ _elm_genlist_item_insert_after(Eo *obj EINA_UNUSED, Elm_Genlist_Data *sd, const
         it->parent->item->items =
           eina_list_append_relative(it->parent->item->items, EO_OBJ(it), eo_after);
      }
+
+   if (after->item->items && after->item->expanded)
+     {
+        eo_after = eina_list_last_data_get(after->item->items);
+        after = efl_data_scope_get(eo_after, ELM_GENLIST_ITEM_CLASS);
+     }
    sd->items = eina_inlist_append_relative
        (sd->items, EINA_INLIST_GET(it), EINA_INLIST_GET(after));