Don't use deprecated QPalette::background()

Summary:
QPalette::background() is deprecated since long time ago. It is advised
to use QPalette::window() instead.

Reviewers: #kwin, apol

Reviewed By: apol

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22373

kcmkwin/kwindecoration/declarative-plugin/previewitem.cpp

@@ -43,7 +43,7 @@ namespace Preview
 PreviewItem::PreviewItem(QQuickItem *parent)
     : QQuickPaintedItem(parent)
     , m_decoration(nullptr)
-    , m_windowColor(QPalette().background().color())
+    , m_windowColor(QPalette().window().color())
 {
     setAcceptHoverEvents(true);
     setFiltersChildMouseEvents(true);