System registry

[jspenger]

There is currently no access to the registry via the PortalsSystem. This was hacked in for the current version of the portalsjs branch, to enable accessing the registry for interactive querying. We should implement this in a cleaner way, and expose it from the PortalsSystem trait as a method with a similar interface as the registry in the builder, with the additional capability to also return everything in the registry.

After some time, it seems that we can do without a Registry for now. We are leaving this issue as discussion, as a discussion topic on whether it is really something that we want to include in the API.