EventObservingCourier is an ObservingCourier that watches Events, of a particular reference root. It automatically registers / un-registers as an observer with the event service.
Construct.
Parameters: location - The key identifying the Portal Page Instance. Parameters: elementId - The key identifying the element on the Portal Page that would need a courier delivered message when things change. Parameters: The - event resource pattern - we watch for only events whose ref start with this.