HV-878 Integrating Optional and JavaFX types

> However, IntegerProperty, DoubleProperty, LongProperty, and FloatProperty all returns Number as a type

Ah, that's interesting. Do you know why it's done that way?

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).