Make 'Navigator.applyCommand()' return boolean to indicate if command was recognized

[dimsuz]

I believe this would make a better API in terms of composing several navigators.

It would be more natural to do something like this:

class TopNavigator(private val childNavigator: Navigator) : Navigator {
  fun applyCommand(command: Command): Boolean {
    if (childNavigator.applyCommand(command)) {
        // child has handled this command, great!
    } else {
       handleCommandInTopNavigator()
    }
}

With the current implementation returning "void" it is not simple to write above logic, you'd have to resort to some "hacky" ways.

If you are not against this suggestion, I could prepare a PR.

[dimsuz]

Hmm... I figured that I don't need childNavigator to implement Navigator in this case. So what I was suggesting is not needed actually :)