efl_ui_widget: move show_region to legacy

efl_ui_widget: move show_region to legacy

summary_:
show_region is simular to focus_region. However, it builds up on setting
custom functions per setter to the widget, which are then executed. This
can be solved better by overwriting the interest_region getter, and
using a function like elm_widget_focus_region_show (This function still
needs to find its place in the eo api).

ref T7553

Depends on D8025

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/D8026