Add $stepForm in PreviousStepInvalidEvent

[roukmoute]

Hi,

I need to retrieve errors from previous step form.
It is not currently possible because PreviousStepInvalidEvent() take current step not the step which contain an error.

So I would like to know, if you accept that I do a P.R. to add new optional argument, to keep backward-compatibility, like this public function __construct(FormFlowInterface $flow, FormInterface $currentStepForm, $invalidStepNumber, FormInterface $invalidStepForm = null) {

What do you think?