[kwin] Register QAbstractItemModel as a qml type in TabBox

It's needed if we don't import any Plasma components.

kcmkwin/kwintabbox/layoutpreview.cpp

@@ -43,6 +43,7 @@ LayoutPreview::LayoutPreview(const QString &path, QObject *parent)
     QQmlComponent *component = new QQmlComponent(engine, this);
     qmlRegisterType<WindowThumbnailItem>("org.kde.kwin", 2, 0, "ThumbnailItem");
     qmlRegisterType<SwitcherItem>("org.kde.kwin", 2, 0, "Switcher");
+    qmlRegisterType<QAbstractItemModel>();
     component->loadUrl(QUrl::fromLocalFile(path));
     QObject *item = component->create();
     auto findSwitcher = [item]() -> SwitcherItem* {

tabbox/tabboxhandler.cpp

@@ -293,6 +293,7 @@ void TabBoxHandlerPrivate::show()
         qmlRegisterType<SwitcherItem>("org.kde.kwin", 2, 0, "Switcher");
         qmlRegisterType<DesktopThumbnailItem>("org.kde.kwin", 2, 0, "DesktopThumbnailItem");
         qmlRegisterType<WindowThumbnailItem>("org.kde.kwin", 2, 0, "ThumbnailItem");
+        qmlRegisterType<QAbstractItemModel>();
     }
     if (m_qmlComponent.isNull()) {
         m_qmlComponent.reset(new QQmlComponent(m_qmlEngine.data()));