elm_panel: fix visibility when switching scrollable mode

elm_panel: fix visibility when switching scrollable mode

Summary:
There are two proplem when switching scrollable mode.
1. a previous layout does not disappear.
2. internal evas_box(sd->bx) should set on "elm.swallow.content" part.

This patch hide remained previous layout and set evas_box to "elm.swallow.content"
part directly.

Thanks to arosis for reporting and sample code.

Test Plan: P262

Reviewers: arosis, eunue, Jaehyun_Cho

Reviewed By: eunue

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D7667