- Change interface to use (CommandSender, List<String> arguments)
- Use CommandArgumentDescription#name instead of "label" (to prevent confusion between command labels and arguments)
- Simplify command difference computation in CommandHandler (no longer consider argument difference)
- Abstract Messages into two layers: the top layer 'Messages' is how Messages can be retrieved and sent as before. In the background, package-private, MessagesManager actually does the file read and worries about I/O while Messages takes care of higher-level things (such as joining two lines or checking the current language).
