Add better validation support for nested fields

[gregmuellegger]

An example syntax:

class MyForm(SuperForm):
    addresses = FormSetForm(...)

    def clean_addresses(self):
        data = self.formsets['addresses'].cleaned_data
        raise ValidationError('...')  # will end up in self.errors['addresses']
        return data  # I would suggest ignoring the returned data as it's not clear were it should end up.

    def formsets_clean(self):
         ... # maybe another solution to clean multiple formsets at once together. The name cannot be clean_formsets as this might clash with a field called "formsets".