Dynamic skeleton validation

[phorward]

Theoretical scribble as an idea?

class TodoSkel(skeleton.Skeleton):

    name = StringBone(
        descr="Name",
    )

    status = SelectBone(
        descr="Status",
        required=True,
        defaultValue="open",
        values={
            "open": "Offen",
            "pending": "In Bearbeitung",
            "closed": "Geschlossen",
        },
    )

    test = StringBone(
        descr="Test",
        required=""" name == "Jan" """,
        validate=("None if len(test) > 10 else 'Too short'", re.compile(r"ab.*")),
        readonly=""" status == "open" """,
        visible=""" status in ("pending", "closed") """,
    )

[phorward]

Relates to #1207