Use itemChange instead of a connect to windowChanged

with itemChange watching SceneChange is more efficient than connecting
to the windowChanged signal, also this won't arrive during teardown,
aoiding the "destructor already ran" assert.

src/scripting/windowthumbnailitem.cpp

@@ -106,8 +106,6 @@ WindowThumbnailItem::WindowThumbnailItem(QQuickItem *parent)
             this, &WindowThumbnailItem::destroyOffscreenTexture);
     connect(Compositor::self(), &Compositor::compositingToggled,
             this, &WindowThumbnailItem::updateFrameRenderingConnection);
-    connect(this, &QQuickItem::windowChanged,
-            this, &WindowThumbnailItem::updateFrameRenderingConnection);
 }
 
 WindowThumbnailItem::~WindowThumbnailItem()
@@ -133,6 +131,14 @@ void WindowThumbnailItem::releaseResources()
     }
 }
 
+void WindowThumbnailItem::itemChange(QQuickItem::ItemChange change, const QQuickItem::ItemChangeData &value)
+{
+    if (change == QQuickItem::ItemSceneChange) {
+        updateFrameRenderingConnection();
+    }
+    QQuickItem::itemChange(change, value);
+}
+
 bool WindowThumbnailItem::isTextureProvider() const
 {
     return true;

src/scripting/windowthumbnailitem.h

@@ -45,6 +45,7 @@ public:
 
 protected:
     void releaseResources() override;
+    void itemChange(QQuickItem::ItemChange change, const QQuickItem::ItemChangeData &value) override;
 
 Q_SIGNALS:
     void wIdChanged();