Welcome page: Set size constraint on the container widget.

Such that QLayout properly handles it. Fixes warnings about
resizing violating size constraints on Windows.

Task-number: QTCREATORBUG-10251

Change-Id: I25361fbc994f23d5616c240663d64f8b3474e7f7
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>

src/plugins/welcome/welcomeplugin.cpp

@@ -170,9 +170,8 @@ WelcomeMode::WelcomeMode() :
     // scrollArea->setWidget(m_welcomePage);
     // scrollArea->setWidgetResizable(true);
 
-    m_welcomePage->setMinimumWidth(880);
-    m_welcomePage->setMinimumHeight(548);
     QWidget *container = QWidget::createWindowContainer(m_welcomePage, m_modeWidget);
+    container->setMinimumSize(QSize(880, 548));
     layout->addWidget(container);
     m_modeWidget->setLayout(layout);