TabBox: Add SwitcherItem to declarative API 3.0

Not sure if this was left out on purpose as a kind of deprecation.

It would be necessary to allow port the tabbox switchers to the newer 3.0 API and use their components (such as `Workspace`)

Probably for `Plasma/5.25` now.

src/kcmkwin/kwintabbox/layoutpreview.cpp

@@ -33,6 +33,8 @@ LayoutPreview::LayoutPreview(const QString &path, bool showDesktopThumbnail, QOb
     QQmlComponent *component = new QQmlComponent(engine, this);
     qmlRegisterType<WindowThumbnailItem>("org.kde.kwin", 2, 0, "ThumbnailItem");
     qmlRegisterType<SwitcherItem>("org.kde.kwin", 2, 0, "Switcher");
+    qmlRegisterType<WindowThumbnailItem>("org.kde.kwin", 3, 0, "WindowThumbnailItem");
+    qmlRegisterType<SwitcherItem>("org.kde.kwin", 3, 0, "TabBoxSwitcher");
     qmlRegisterType<QAbstractItemModel>();
     component->loadUrl(QUrl::fromLocalFile(path));
     if (component->isError()) {

src/tabbox/tabboxhandler.cpp

@@ -291,6 +291,7 @@ void TabBoxHandlerPrivate::show()
 #ifndef KWIN_UNIT_TEST
     if (m_qmlContext.isNull()) {
         qmlRegisterType<SwitcherItem>("org.kde.kwin", 2, 0, "Switcher");
+        qmlRegisterType<SwitcherItem>("org.kde.kwin", 3, 0, "TabBoxSwitcher");
         m_qmlContext.reset(new QQmlContext(Scripting::self()->qmlEngine()));
     }
     if (m_qmlComponent.isNull()) {