[RFC]Event Emitter Chain

[mortalYoung]

Background

It was started from a bug about the users couldn't stop the default behavior of closing tab, because we defined it in Molecule, and users couldn't interfere with it. There were two ways to resolve it.

1. Remove it from Molecule, which involved with too many break changes
2. Event Emitter Chain

Every event starts with on in Molecule are all based on event emitter. And events are just like chains pending to executed.

Proposals

1. Keep the same of most common event
2. If there was a event which want to stop the executed chain, it should have a way to stop it. For example, call this.stopDelivery()

Changes

1. Call this.stopDelivery() to stop the following event executed.
2. In async function, should call this.async() first to call event runner waitting for current event.

[mortalYoung]

It's unnecessary to call this.aynsc(), as be more used to use await in listener.