This adds a service that listens for changes in the UI state and
the set of loaded annotations and generates a thread structure in
response, which can then be visualized by a view.

This commit also adds a first integration test which wires together
the non-visual parts of the new threading implementation.

 * Add 'annotations' to the UI state in annotationUI and update
   this when annotations are loaded or unloaded.

 * Add rootThread which listens for changes in the UI state and
   generates a new conversation thread structure in response.

 * Add an integration test which tests the wiring of the non-visual
   parts of the new threading implementation.