IZPACK-1197 - Variable and dynamic variable improvements

This is a compound pull request for a package of subissues regarding https://jira.codehaus.org/browse/IZPACK-1197:

Enhancements of dynamic variables (http://docs.codehaus.org/display/IZPACK/Dynamic+Variables) and their subsequent improvements is one of the major features introduced with IzPack 5.0.
Nevertheless, there is a couple of issues left with the goal of using them more smoothly.
A special focus is integrating static and dynamic variables as much as possible.

There are the following subissues available:
- https://jira.codehaus.org/browse/IZPACK-1199
- https://jira.codehaus.org/browse/IZPACK-1200
- https://jira.codehaus.org/browse/IZPACK-1201

They are bound to one super-issue, because they address one and the same topic - make variable handling in IzPack more comfortable and consistent.