efl_ui_widget: make interest_region_mode legacy

efl_ui_widget: make interest_region_mode legacy

summary_:
interest_region_mode makes it possible, that interest_region_get either
returns WIDGET or ITEM coordinates. However, efl-api does not have
items. That means, this can be legacy.

ref T7553

Depends on D8026

Reviewers: woohyun, cedric, Jaehyun_Cho, zmike, segfaultxavi

Reviewed By: segfaultxavi

Subscribers: #reviewers, #committers

Tags: #efl

Maniphest Tasks: T7553

Reviewed-by: Xavi Artigas <xavierartigas@yahoo.es>
Differential Revision: https://phab.enlightenment.org/D8027