`max` value has been prepared to contain a `null` value, however,
the constructor with its default value of `max = 0` prevents
`$max` to ever be `null`.

Hence, declare `$max` as `int` instead of `?int`.

Up until now, it has been very hard to determin whether a progressbar
has a known maximum value or not, because the property `max` is an
integer. Up until now, `max == 0` has been treated like we do _not_
know the `max` value. However, `max == 0` _is_ a valid `max` value
when iterating over an empty but countable array for example.

We cannot simply keep `max == null` if unknown, because that would break
rendering mechanics and fixing those would require interface changes.

Hence, enter `isMaxKnown` - a property which keeps track of whether the
`max` value is known or not.