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/D10386

lookandfeel.dark/contents/defaults

@@ -19,5 +19,5 @@ LayoutName=org.kde.breeze.desktop
 [kwinrc][DesktopSwitcher]
 LayoutName=org.kde.breeze.desktop
 
-[kwinrc][WindowDecoration]
+[kwinrc][org.kde.kdecoration2]
 library=org.kde.breeze