Use a real event for reload notification

Don't rely on the hacky state.allLoaded variable, but us a $rootScope
event instead.
Add fn to replace state through a single call

As in moving to a next sentence in a document. Triggers a stateReload
event.
Add navigator Ctrl and service

Very very simple right now, subject to change once a second (slightly
different) retriever is tried out.
Use navigator in treebankRetriever
Minor restyling of NavCtrl html
Refactor navigator service