Added (basic) Recode-function.
The function can recode "short text"/"long text"-variables. Additional
variables ("integers" and "booleans") may be selected and can be recoded
whenever the change to the String-based variable calls for it.

Major open tasks: 
1) make sure that "integer"/"boolean" variables can be recoeded as well
(and not just as additional variables)
2) make sure that "short text"/"long text"-variables can be recoded as
additional variables.