Event tree building / prep for activity pane tree view

WIP: Not everything here is integrated yet. The main victory is the build tree method, along with prep work. Still to do:
- Pipe event tree into ActivityNode update method
- Build out ActivityTree view
- Install ActivityTree view in event panel

Progress towards #103
```js
hasListeners: function() {
  return this.filter(function(event) {
    return event. get('listeners').length
  });
}
```