elm_box: Fix miscalculated items' size issue in homogeneous box

elm_box: Fix miscalculated items' size issue in homogeneous box

Summary:
When box has expanded items (evas_object_size_hint_weight_set() with EVAS_HINT_EXPAND)
and homogeneous is set (elm_box_homogeneous_set() with EINA_TRUE),
there is a bug that it miscalculates its items size omitting padding size (space among items),
so the last item looks cropped or having smaller size than others.
This patch makes box calculate its items' size correctly.

T2033

Reviewers: Hermet, seoz

Subscribers: seoz, Hermet

Maniphest Tasks: T2033

Differential Revision: https://phab.enlightenment.org/D2016