Added basic UI for Nestable Async Events

This CL adds basic UI support for nestable async events.
Still need to find out whether the UI looks okay. But right
now it looks like the picture in the design doc below.

Design Doc:
https://docs.google.com/document/d/1Z2uqj59UEts5IiXX78mkdU4kd6e7kE3JUKPoDK97bVs/edit?usp=sharing
addressed comments
added another test case
Incorporated suggestions in parsing rules
a few style fixes
move warning message
updated tests
addressed comments
Merge pull request #617 from xunjieli/nestableAsyncUI

Added basic UI for Nestable Async Events