Support `nullable = false` for properties

[pbor]

If an object has a string property, getters always return Option<String>. If the property is always set by construction, it would be great to be able to tell gir that it is not nullable, eg:

    [[object.property]]
    name = "my-prop"
    nullable = false

[sdroege]

@pbor Do you want to give that a try? That should be relatively straightforward to add as a) we already read the nullable annotation for properties, b) we have nullable configuration for other things. That should be possible to handle basically the same way