Obsolete / replace / document several features beginning from IzPack 5.0 which might break existing IzPack < 5.0 environments

There are several features in IzPack which could be replaced by their smarter counterparts.
Example:

• the os attribute ("unix", "windows", "macosx") has already the cleaner alternative of the nested <os family="..."/> tag
• the VariableExistence condition has been replaced by the Exists condition, which accepts more object types to test for existance than only variables.

Since those changes might break previous installation descriptions and require to adapt them this should be well-marked in the documentation.