Override QMainWindow::setCentralWindow() to make it private

The central widget is used internally by KDDW to make all of this
possible. It shouldn't be used by users

src/MainWindow.cpp

@@ -79,3 +79,8 @@ MainWindow::~MainWindow()
 {
     delete d;
 }
+
+void MainWindow::setCentralWidget(QWidget *w)
+{
+    QMainWindow::setCentralWidget(w);
+}

src/MainWindow.h

@@ -46,6 +46,8 @@ public:
     ~MainWindow() override;
 
 private:
+    using QMainWindow::setCentralWidget;
+    void setCentralWidget(QWidget *); // overridden just to make it private
     class Private;
     Private *const d;
 };