Implement didSet and willSet observable properties

[angelolloqui]

In Swift, as part of the language, a property can have a didSet and/or willSet. Example:

class A {
    var isLocating = false {
        didSet {
            delegate.set(isLocating: isLocating) 
        }
    }
}

In Kotlin there is no default way of expressing property observers but can be done with delegated properties or by writing a custom setter (https://stackoverflow.com/a/39842353/378433)

For simplicity, implement a custom setter like:

class A {
    var isLocating = false
        set(value) {
            field = value
            delegate.set(isLocating = value)
        }
}

In case of willSet, just write the code before the field = value assignment

[angelolloqui]

Implemented in 8d6c593