QMessageBox should use TaskDialog on Vista/Windows 7

On Windows Vista/Windows 7, the task dialog maps much more closely to the functionality provided by QMessageBox. To generate a message as shown in the UX guidelines (http://msdn.microsoft.com/en-us/library/aa511263(v=MSDN.10).aspx), Qt should simply map ...

setText == pszMainInstruction
setInformativeText = pszMainContent

This is consistent with the current documentation of informativeText ("Infromative text can be used to expand upon the text() to give more information to the user. On the Mac, this text appears in small system font below the text(). On other platforms, it is simply appended to the existing text.") On Windows Vista/7, it would simply trigger the use of TaskDialog.