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
It looks like the only time children can be undefined is when you do the initial assignment from slice.subSlices, yea? If so, can we just put a check before the while and do something like:
```
if (children === undefined)
  continue;
while (children.length > 0) {
 ...
}
```