HV-878 Integrating Optional and JavaFX types

> Ah, that's interesting. Do you know why it's done that way?

They're all subclasses of `ReadOnlyProperty<Number>`.

> I don't really think there will be a problem though, as we e.g. support @Max or @Min for Number anyways (@Size is only for collection sizes and string lengths).

Right. I got my constraints mixed up.