Add a "save and goto" function

[bartTC]

Currently we have a "save and goto next step" function, and a "goto step" function.

If I go back to a previous step and change data and go directly to a step, the form data is not saved, I have to use the classic "Continute to next step" behaviour.

We need a additonal "save and goto step" function.

[stephrdev]

could be done by overwriting the process_post_request.