Fix warning about non-NOTIFYable property

src/MainWindow.h

@@ -61,10 +61,10 @@ class MainWindow : public QMainWindow
 {
         Q_OBJECT
 
-        Q_PROPERTY(int x READ x)
-        Q_PROPERTY(int y READ y)
-        Q_PROPERTY(int width READ width)
-        Q_PROPERTY(int height READ height)
+        Q_PROPERTY(int x READ x CONSTANT)
+        Q_PROPERTY(int y READ y CONSTANT)
+        Q_PROPERTY(int width READ width CONSTANT)
+        Q_PROPERTY(int height READ height CONSTANT)
 
 public:
         explicit MainWindow(QWidget *parent = nullptr);