Button back-forward to pass to

[davidzbiral]

We talked that by scrolling or PgUp/PgDn the annotator it is difficult to trigger changing of the territory tree unfolded. But instead, we will do a Previous and Next territory button, which will trigger the normal action for opening that territory, i.e. open Annotator at the opening anchor of this (sub)territory, and also unfold the territory tree and statement list at the right place.