Fix LnF theme data for window decoration

Summary:
Plasma LnF KCM reads the window decoration data from
    cg = KConfigGroup(conf, "kwinrc");
    cg = KConfigGroup(&cg, "org.kde.kdecoration2");

Reviewers: mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10384