CPack: Add an "IFW" generator for Qt Framework Installer

Add support for packaging with the Qt Framework Installer tools:

 http://qt-project.org/doc/qtinstallerframework/index.html

Reviewed-by: Nils Gladitz <nilsgladitz@gmail.com>
CPackIFW: Replace use of strftime with cmTimestamp

The latter is more portable.
Merge topic 'cpack-ifw-generator' into next

a943d41d CPackIFW: Replace use of strftime with cmTimestamp
a9006316 CPack: Add an "IFW" generator for Qt Framework Installer