Add action delegation

[jnicklas]

In #90 we added filter delegation, in #102 we proposed locator delegation. The final piece of the puzzle is action delegation! Action delegation kind of already exists today:

HTML.extend('calendar')
  .selector('.calendar')
  .actions({
    async open(interactor) {
      await interactor.find(Button('Open').click());
    }
  });

The idea here is to align this with the filter syntax, to achieve something like the following:

HTML.extend('calendar')
  .selector('.calendar')
  .actions({
    open: Button('Open').click()
  });

Which would have identical behaviour.

The problem is that actually implementing this is probably very difficult, and will require some deep rethinking of interactions to make this possible.