`_root` is calculated dynamically based on the clip the currently executing function was called in.
Other things that used `context.root` have been changed to either update all layers or just update layer 0, which is the former `context.root`.
Interestingly enough, very little actually has to be done inside the async process for XML. The async process basically just fetches data and fires an event handler when it's done. Everything else is handled via a system builtin, `XML.onData`.
This also adjusts `MovieClip.unloadMovie` to do just that, instead of removing the clip from the display list. We also have to unload clips when loading new movies into them, since `unloadMovie` desugars to loading `""` as the URL.
David Wendt