multibuttonentry : fix box indicated item correctly in item_insert_before and item_in...

multibuttonentry : fix box indicated item correctly in item_insert_before and item_insert_after

Summary:
If user added item with item_insert_before and item_insert_after, item was not displayed.
For this reason, fixed box indicated item correctly from reference->button to VIEW(reference).
Additionally, remove the unused variable button in _Multibuttonentry_Item struct.

Reviewers: woohyun, CHAN, cedric, jpeg

Reviewed By: CHAN

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D4708