Ensure the accepted state of mouse events is passed to QWindowSystemInterface.

On Windows, Qt needs to return false in case it does not handle the "extra"
buttons (like WM_XBUTTONDOWN) which causes Windows to send the
corresponding WM_APPCOMMAND message (like APPCOMMAND_BROWSER_FORWARD).

Task-number: QTBUG-48117
Change-Id: I093cd2d8205a39c3a042dd8a0d19c7229003761f
Reviewed-by: Andy Shaw <andy.shaw@theqtcompany.com>

src/gui/kernel/qguiapplication.cpp

@@ -1809,6 +1809,7 @@ void QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::Mo
     }
 
     QGuiApplication::sendSpontaneousEvent(window, &ev);
+    e->eventAccepted = ev.isAccepted();
     if (!e->synthetic() && !ev.isAccepted()
         && !frameStrut
         && qApp->testAttribute(Qt::AA_SynthesizeTouchForUnhandledMouseEvents)) {

src/widgets/kernel/qwidgetwindow.cpp

@@ -552,6 +552,7 @@ void QWidgetWindow::handleMouseEvent(QMouseEvent *event)
         translated.setTimestamp(event->timestamp());
         QApplicationPrivate::sendMouseEvent(receiver, &translated, widget, m_widget,
                                             &qt_button_down, qt_last_mouse_receiver);
+        event->setAccepted(translated.isAccepted());
     }
 #ifndef QT_NO_CONTEXTMENU
     if (event->type() == contextMenuTrigger && event->button() == Qt::RightButton