transient window manual test: set window type to Dialog

On X11, setting the transient parent is not enough to get it
centered w.r.t. its parent: it must also be a dialog window.

Change-Id: Icfc664e17e53f23cd025dead30e3966f859a1dc5
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

tests/manual/transientwindow/mainwindow.cpp

@@ -53,6 +53,7 @@ void MainWindow::toggleVisible()
         m_window->setTransientParent(windowHandle());
         m_window->setMinimumSize(QSize(200, 100));
         m_window->setTitle("Transient Window");
+        m_window->setFlags(Qt::Dialog);
     }
     m_window->setVisible(!m_window->isVisible());
 }