Get rid of CopyOnWriteArrayList

[neonowy]

Replaces java.util.concurrent.CopyOnWriteArrayList with a lightweight wrapper around HashSet for storing listeners in a thread-safe way. (See #1892 (comment))

[westnordost]

Looks good, thank you!

I figure, all this

    fun addListener(listener: Listener) {
        listeners.add(listener)
    }
    fun removeListener(listener: Listener) {
        listeners.remove(listener)
    }

stuff could be removed now too and replaced with the listeners field to be public, now that it does not expose the whole of a mutable list. (I.e. users call someController.listeners.add(...))

To be clean, it would be necessary however to change Listeners into an interface and make a class like HashSetListeners or whatever that implements Listeners so that users of Listeners don't have access to the forEach method.

So, not sure if it would be worth it 🤷, just thinking aloud. This PR is approved, anyway.