Set Qt::AA_DontCreateNativeWidgetSiblings on Windows/Mac.

Prevent creation of native windows for subwidgets, caused
by the welcome screen.

Task-number: QTCREATORBUG-10251

Change-Id: I2b475e963c5dfd1485f0876817c88d80fc2abd25
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
Fix welcomescreen project file

Task-number: QTCREATORBUG-10257

Change-Id: If12f3ee18dda18da13fe2ea39af42cae4fe21ddf
Reviewed-by: Robert Loehning <robert.loehning@digia.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@digia.com>
QmlProjectManager: Fix compiler warning about unused variable.

Change-Id: I8a8c727fca12fc5ad67e8137f0c730be66e445bd
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>