Fix spelling of attributes added by commit v3.6.0-rc1~52^2 (CPackIFW: Add support for Promoting Updates, 2016-05-17).
Add support for this feature added by QtIFW 2.0.3: http://doc.qt.io/qtinstallerframework/ifw-updates.html Add a `cpack_ifw_update_repository` command as porcelain.