Fix spacing in "Select test picture:" row

Summary:
Use QBoxLayout::addLayout() instead of QLayout::addItem()
to add a child layout. It's more preferable and makes Qt use a
correct spacing for the child layout.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D5882