cubicweb #1534836 [formfields] fromcardinality should preset the required attribute [open]

current:

@staticmethod
def fromcardinality(card, **kwargs):
    kwargs.setdefault('widget', fw.Select(multiple=card in '*+'))
    return RelationField(**kwargs)

proposed:

@staticmethod
def fromcardinality(card, **kwargs):
    kwargs.setdefault('required', card in '1+'))
    kwargs.setdefault('widget', fw.Select(multiple=card in '*+'))
    return RelationField(**kwargs)
prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>