shell/panelview: Register enums for QML

So MoreSettingsMenu.qml does not have to use numbers to
represent VisibilityMode and OpacityMode anymore.

shell/panelview.cpp

@@ -94,6 +94,9 @@ PanelView::PanelView(ShellCorona *corona, QScreen *targetScreen, QWindow *parent
     m_strutsTimer.setSingleShot(true);
     connect(&m_strutsTimer, &QTimer::timeout, this, &PanelView::updateStruts);
 
+    // Register enums
+    qmlRegisterUncreatableMetaObject(PanelView::staticMetaObject, "org.kde.plasma.shell.panel", 0, 1, "Global", QStringLiteral("Error: only enums"));
+
     qmlRegisterAnonymousType<QScreen>("", 1);
     rootContext()->setContextProperty(QStringLiteral("panel"), this);
     setSource(m_corona->kPackage().fileUrl("views", QStringLiteral("Panel.qml")));