Handle progressive loading of resources.
A component that listens to this message must do the following:
- Increase *msg.total by the non-zero number of loading tasks this component can perform.
- If *msg.progressed == true, return and do nothing.
- If you've loaded everything, increase *msg.progress by the value you added to .total
- Otherwise do some loading, set *msg.progressed = true, and increase *msg.progress by a value indicating how much progress you've made in total (0 <= p <= what you added to .total) In some situations these messages will never be sent - components must ensure they load all their data themselves before using it in that case.