Elevate "scene" and "subtasks" to be properties in task.vwf.yaml

Also cleaned up some formatting.
Add a sourceURL to humvee.vwf.yaml
Create an app$ property for a couple of children in humvee-lesson/lesson.vwf.yaml
Replace humvee-lesson app$ property w/ scene property inherited from task.vwf.yaml
Add test for flushEventListeners to make sure kernel and driver stay in sync
Fix failing test: flushEventListeners now keeps kernel and driver in sync