do not misuse QGroupBox as a layout helper, instead use a layout directly

Summary:
QGB with some styles has very unfortunate rendering if it has no title
nor frame configured. e.g. with the windows 9x style it renders the top
border line assuming there is a title even when there is none.
Ultimately we do not need QGB here at all though as we have it entirely
hidden and only use it as glorified layout container. So, replace QGB with
a Layout directly. Reduces excess code, memory footprint and fixes
appearance with various styles.

(technically we could put the entire setup into the designer file and
 do away with the QButtonGroup as radios are autoexclusive with other
 radios in the same parenting qwidget anyway; seems unncessarily invasive
 though)

Test Plan: no more silly line artifact with windows 9x style; breeze continues to look stelar

Reviewers: sandsmark, adridg

Reviewed By: adridg

Subscribers: adridg, kde-utils-devel

Differential Revision: https://phabricator.kde.org/D8941