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 :-)
Those changes seems  to be not linked to this pull-request.
In the future, please do a separate PR this will be easier for  reviewer/committer :).
(and patch could be integrated sooner ;))