Add virtual QWindow::closeEvent handler

The default implementation does nothing; the processing of accepted
close events remains in the QWidget::event handler, so that subclasses
don't have to call the super class in order to free window system
resources and emit lastWindowClosed signals.

QWidgetWindow::event is reimplemented to handle QEvent::Close as well,
calling QWidgetPrivate::close_helper, which then delivers a separate
QCloseEvent to the widget.

The order of execution for widgets is after this change:

1) QWidgetWindow::event
2) QWidgetWindow::handleCloseEvent (calls QWidget::event/closeEvent)
3) QWindow::event
4) QWindow::closeEvent <- does nothing, not overridden
5) default cleanup handling in QWindow::event

and for Qt Quick after the corresponding change in qtdeclarative:

1) QQuickWindow::event
2) QWindow::event
3) QQuickWindow::closeEvent <- emits closed
4) default cleanup handling in QWindow::event

[ChangeLog][QtGui][QWindow] closeEvent has been added as a virtual
function.

Task-number: QTBUG-31019
Change-Id: I201f5ee9c6a73b949986648e3bd288d2c7898f28
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/gui/kernel/qwindow.cpp

@@ -2234,7 +2234,7 @@ void QWindow::showNormal()
     quitting the application. Returns \c true on success, false if it has a parent
     window (in which case the top level window should be closed instead).
 
-    \sa destroy(), QGuiApplication::quitOnLastWindowClosed()
+    \sa destroy(), QGuiApplication::quitOnLastWindowClosed(), closeEvent()
 */
 bool QWindow::close()
 {
@@ -2319,6 +2319,19 @@ void QWindow::hideEvent(QHideEvent *ev)
     ev->ignore();
 }
 
+/*!
+    Override this to handle close events (\a ev).
+
+    The function is called when the window is requested to close. Call \l{QEvent::ignore()}
+    on the event if you want to prevent the window from being closed.
+
+    \sa close()
+*/
+void QWindow::closeEvent(QCloseEvent *ev)
+{
+    Q_UNUSED(ev);
+}
+
 /*!
     Override this to handle any event (\a ev) sent to the window.
     Return \c true if the event was recognized and processed.
@@ -2395,6 +2408,7 @@ bool QWindow::event(QEvent *ev)
 #endif
 
     case QEvent::Close:
+        closeEvent(static_cast<QCloseEvent*>(ev));
         if (ev->isAccepted()) {
             Q_D(QWindow);
             bool wasVisible = isVisible();

src/gui/kernel/qwindow.h

@@ -69,6 +69,7 @@ class QMoveEvent;
 class QResizeEvent;
 class QShowEvent;
 class QHideEvent;
+class QCloseEvent;
 class QKeyEvent;
 class QMouseEvent;
 #if QT_CONFIG(wheelevent)
@@ -350,7 +351,7 @@ protected:
 
     virtual void showEvent(QShowEvent *);
     virtual void hideEvent(QHideEvent *);
-    // TODO Qt 6 - add closeEvent virtual handler
+    virtual void closeEvent(QCloseEvent *);
 
     virtual bool event(QEvent *) override;
     virtual void keyPressEvent(QKeyEvent *);