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
I'm not sure I fully understand what's going on here. Why do we take  the params out of the arguments and then put them directly into the hash with the arguments? But, we don't do that if one of the items has a undefined params?