`BaseBone.isInvalid()` broken and incomplete

[phorward]

The isInvald() respectively vfunc-parameter to BaseBone is incomplete and broken.
This is what I wanted to achieve:

    roles = SelectBone(
        descr=i18n.translate("viur.user.bone.roles", defaultText="Roles"),
        values=conf["viur.user.roles"],
        required=True,
        multiple=True,
        vfunc=lambda values:
            i18n.translate(
                "user.bone.roles.invalid",
                defaultText="Invalid role setting: 'custom' can only be set alone.")
            if "custom" in values and len(values) > 1 else None,
        defaultValue=list(conf["viur.user.roles"].keys())[0],
    )

There are multiple problems here:

1. vfunc/isInvalid() is generally not implemented for SelectBone
2. The vfunc/isInvalid()-function in StringBone is implemented on single values only
3. In this case, I want to verify the complete bone value and reject, if it contains "custom" as the role name, as "custom" may only be used separately as a special role. It's impossible to fix this.

That's all for now.

[phorward]

The described problem is part of #736