feat(editor): K on a track is a break of the grade, as a point of a road: the track split at the cursor, the point between the two at the height of the ground and the ballast there, each of them an even grade from its other end up to it

Selftest kbreak: K on the track nearest a point, logs the joint against the ground.

application/editormodeselftest.cpp

@@ -65,6 +65,7 @@ editor_mode::selftest_result editor_mode::selftest_dispatch(std::string const &c
 //   camera <x> <y> <z> <x> <y> <z>        from the first point, looking at the second
 //   select <track name>
 //   selectat <x> <z>                     the path nearest the point, within 3 m, as a click on it
+//   kbreak <x> <z>                       K on the track nearest the point: split there, a break of the grade at the height of the ground
 //   blade [name]                         logs the blades of each switch cut from the rails: tip, 50 mm head, full head, heel, drop of the head, open flangeway, the gauge lines apart against x^2/2R, vertices in both positions
 //   frog [name]                          logs where the inner rails of each switch cross: distance on both paths, 1:n, throat, nose, check rails
 //   guards [name]                        logs each guard rail and what each of its ends and breaks is cut along; a fault when a cut is not along a rail or a glued end not on the guard rail of the other track