feat(editor): the terrain model loads from a clearer panel and follows the tracks edited

The panel gives the place on the map or typed, the area, one main button, a
progress bar with Stop and a banner over the view while it loads. The terrain
grown ahead of a track goes on at the height of the one beside it and skips
the ground made by hand. The formations follow the tracks moved, laid and
removed, in the same step of the history; a whole load is one step, the
ground grown ahead none.

editor/editorTrack.hpp

@@ -506,6 +506,7 @@ class editor_track
 		virtual ~observer() = default;
 		virtual void track_captured(TTrack const &Track) = 0;
 		virtual void tracks_committed(std::vector<TTrack *> const &Tracks) = 0;
+		// still under its name
 		virtual void track_retired(TTrack &Track) = 0;
 		// called before the commit; Original keeps the beginning of the path, given in First
 		virtual void track_split(TTrack &Original, TTrack &Created, segment_data const &First) = 0;