Reconsider `onEvent` name

[DavyLandman]

Perhaps it should be on on per

[DavyLandman]

var watcherSetup = Watcher.watch(directory, WatchScope.PATH_AND_CHILDREN)
  .withExecutor(Executors.newCachedThreadPool()) // optionally configure a custom thread pool
  .onEvent(watchEvent -> {
    System.err.println(watchEvent);
  });

vs

var watcherSetup = Watcher.watch(directory, WatchScope.PATH_AND_CHILDREN)
  .per(watchEvent -> {
    System.err.println(watchEvent);
  });

or even:

var watcherSetup = Watcher.watch(directory, WatchScope.PATH_AND_CHILDREN)
  .onModify(mod -> {
    System.err.println(mod);
  });

[DavyLandman]

Conclusion after discussion: onEvent should become on with a possible overload that accepts a visitor like interface so that user can only get a callback for a certain kind of event.