PaintedWindow example: Use standard window flags.

Change-Id: Ie7202efc88c176e2c39574677227c9f177c8e6d1
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

examples/opengl/paintedwindow/paintedwindow.cpp

@@ -56,7 +56,7 @@ PaintedWindow::PaintedWindow()
     format.setSamples(4);
 
     setSurfaceType(QWindow::OpenGLSurface);
-    setFlags(Qt::Window | Qt::WindowTitleHint | Qt::WindowMinMaxButtonsHint | Qt::WindowCloseButtonHint);
+    setFlags(Qt::Window | Qt::WindowTitleHint | Qt::WindowSystemMenuHint | Qt::WindowMinMaxButtonsHint | Qt::WindowCloseButtonHint);
     setFormat(format);
 
     create();