Do not use QWindowsFileDialogHelper for Windows Server 2003

Windows Server 2003 is based on Windows XP and should use
QWindowsXpFileDialogHelper as it does not support the CLSID-based
IFileDialog interfaces that are available from Windows Vista onwards.

Change-Id: Idd973f9ec4c98d1f2fb7e835de64532edeccfc72
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>