appmenu: Use existing menu in compact represenation

There's no need to reconstruct the entire menu, when it exists already.
Also shows correct behavior on Wayland because the imported menues have
correct parents compared to the new  one.
BUG:438467

applets/appmenu/lib/appmenuapplet.cpp

@@ -125,14 +125,10 @@ QMenu *AppMenuApplet::createMenu(int idx) const
     QAction *action = nullptr;
 
     if (view() == CompactView) {
-        menu = new QMenu();
-        for (int i = 0; i < m_model->rowCount(); i++) {
-            const QModelIndex index = m_model->index(i, 0);
-            const QVariant data = m_model->data(index, AppMenuModel::ActionRole);
-            action = (QAction *)data.value<void *>();
-            menu->addAction(action);
+        auto menuAction = static_cast<QAction*>(m_model->data(QModelIndex(), AppMenuModel::ActionRole).value<void *>());
+        if (menuAction) {
+            menu = menuAction->menu();
         }
-        menu->setAttribute(Qt::WA_DeleteOnClose);
     } else if (view() == FullView) {
         const QModelIndex index = m_model->index(idx, 0);
         const QVariant data = m_model->data(index, AppMenuModel::ActionRole);

applets/appmenu/plugin/appmenumodel.cpp

@@ -234,12 +234,20 @@ QList<QAction *> AppMenuModel::flatActionList()
 
 QVariant AppMenuModel::data(const QModelIndex &index, int role) const
 {
-    const int row = index.row();
-    if (row < 0 || !m_menuAvailable || !m_menu) {
+    if (!m_menuAvailable || !m_menu) {
         return QVariant();
     }
 
+    if (!index.isValid()) {
+        if (role == MenuRole) {
+            return QString();
+        } else if (role == ActionRole) {
+            return QVariant::fromValue((void *)(m_menu->menuAction()));
+        }
+    }
+
     const auto actions = m_menu->actions();
+    const int row = index.row();
     if (row == actions.count() && KWindowSystem::isPlatformWayland()) {
         if (role == MenuRole) {
             return m_searchAction->text();