Add support for observing resources from within web UI

I have added initial support for starting observation of resources from within the web UI. So far you can only start to observe, but not cancel an existing observation. However, notifications sent from the client for the observed resources are reflected in the web UI, i.e. the values are dynamically updated :-)
Hmm, I was under the impression that the browser uses long polling for receiving the events. However, having dug a little deeper into the EventSource specification and the event-stream format I now see your point. The browser is not supposed to finish the request and spawn a new one but instead keep the original request open and continuously receive events on that connection. In this case it doesn't make sense to complete the Continuation but instead keep it "open" forever ... I will take a look at that and fix this.