Don't use AppMenuModel in AppMenuApplet

That's in a plugin we don't link against here, we only see its header.
This happens to work with Qt5, but Qt6 wants fully defined types for
properties. Use a QAbstractItemModel pointer instead, we only access that
API anyway, and for QML the difference doesn't matter.

applets/appmenu/lib/appmenuapplet.cpp

@@ -67,12 +67,12 @@ void AppMenuApplet::init()
 {
 }
 
-AppMenuModel *AppMenuApplet::model() const
+QAbstractItemModel *AppMenuApplet::model() const
 {
     return m_model;
 }
 
-void AppMenuApplet::setModel(AppMenuModel *model)
+void AppMenuApplet::setModel(QAbstractItemModel *model)
 {
     if (m_model != model) {
         m_model = model;
@@ -290,3 +290,4 @@ bool AppMenuApplet::eventFilter(QObject *watched, QEvent *event)
 K_PLUGIN_CLASS_WITH_JSON(AppMenuApplet, "../package/metadata.json")
 
 #include "appmenuapplet.moc"
+#include "moc_appmenuapplet.cpp"

applets/appmenu/lib/appmenuapplet.h

@@ -7,17 +7,18 @@
 #pragma once
 
 #include <Plasma/Applet>
+
+#include <QAbstractItemModel>
 #include <QPointer>
 
 class QQuickItem;
 class QMenu;
-class AppMenuModel;
 
 class AppMenuApplet : public Plasma::Applet
 {
     Q_OBJECT
 
-    Q_PROPERTY(AppMenuModel *model READ model WRITE setModel NOTIFY modelChanged)
+    Q_PROPERTY(QAbstractItemModel *model READ model WRITE setModel NOTIFY modelChanged)
 
     Q_PROPERTY(int view READ view WRITE setView NOTIFY viewChanged)
 
@@ -41,8 +42,8 @@ public:
     QQuickItem *buttonGrid() const;
     void setButtonGrid(QQuickItem *buttonGrid);
 
-    AppMenuModel *model() const;
+    QAbstractItemModel *model() const;
-    void setModel(AppMenuModel *model);
+    void setModel(QAbstractItemModel *model);
 
     int view() const;
     void setView(int type);
@@ -69,6 +70,6 @@ private:
     int m_viewType = FullView;
     QPointer<QMenu> m_currentMenu;
     QPointer<QQuickItem> m_buttonGrid;
-    QPointer<AppMenuModel> m_model;
+    QPointer<QAbstractItemModel> m_model;
     static int s_refs;
 };