Multi stage add forms

[nils-van-zuijlen]

Is your feature request related to a problem? Please describe.
I have some complex validation on a model, that would be greatly simplified by doing it in stages and only allowing the user to edit parts of the model at a time.

This can be partly solved for created objects, where I can define most fields as readonly in the change_view and add actions with dedicated forms to change the other fields.

However, multiple successive forms are not available at creation time, which would require me to define a stripped down creation form and then require the user to go edit the object through the actions to define it totally.

Describe the solution you'd like
I would like to be able to define a form stackup for object creation.

Describe alternatives you've considered
See above.

Additional context
The model causing these problems has both a simple ManyToMany and a complex ManyToMany Inline that must agree together. Currently, it is possible to get the validators in a quasi-unrecoverable state because the ones on the Inline's FormSet and the ones on the Admin's Form do not have access to each other's data, so they check against the database, which can be outdated.

More context available on request during spk working hours.