Revamp Mandelbrot example: Fix Includes

Reorder the includes following the coding conventions.

Remove unneeded includes.

Add needed includes to avoid Transitive includes.

QGestureEvent is passed by a pointer in the gestureEvent method in
mandelbrotwidget.h. So, we do not really need to include the header.
Instead, forward-declare QGestureEvent class in a namespace.

Task-number: QTBUG-108861
Pick-to: 6.5
Change-Id: I48cfd663bf98350281ef7f31e6c0ef3e301a08ca
Reviewed-by: Dennis Oberst <dennis.oberst@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit a4d7a4ccb39daa86ee2ef23b1408344822ae7c16)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/corelib/threads/mandelbrot/main.cpp

@@ -2,15 +2,12 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
 #include "mandelbrotwidget.h"
+#include "renderthread.h"
 
 #include <QApplication>
-
-#include <QScreen>
-
 #include <QCommandLineParser>
 #include <QCommandLineOption>
 #include <QDebug>
-#include <QRect>
 
 using namespace Qt::StringLiterals;
 

examples/corelib/threads/mandelbrot/mandelbrotwidget.cpp

@@ -4,6 +4,7 @@
 #include "mandelbrotwidget.h"
 
 #include <QGesture>
+#include <QGestureEvent>
 #include <QKeyEvent>
 #include <QPainter>
 

examples/corelib/threads/mandelbrot/mandelbrotwidget.h

@@ -4,11 +4,14 @@
 #ifndef MANDELBROTWIDGET_H
 #define MANDELBROTWIDGET_H
 
-#include <QGestureEvent>
-#include <QPixmap>
-#include <QWidget>
 #include "renderthread.h"
 
+#include <QPixmap>
+#include <QWidget>
+
+QT_BEGIN_NAMESPACE
+class QGestureEvent;
+QT_END_NAMESPACE
 
 //! [0]
 class MandelbrotWidget : public QWidget

examples/corelib/threads/mandelbrot/renderthread.cpp

@@ -4,7 +4,6 @@
 #include "renderthread.h"
 
 #include <QImage>
-
 #include <QElapsedTimer>
 #include <QTextStream>