refactor(editor): the speed check is a tool of its own

The second tool out of editor_mode. Tools get two more hooks, the overlay
drawn over the view and the options in the tool window, and the editor
gives them the track it focuses on, the node it makes current, the norms
of the route design and the snapshot of the tracks for the undo.

The check gets a self-test command, speedcheck <scope>, which runs it
through and logs the paths found too fast for their geometry.

application/editormode.cpp

@@ -1466,13 +1466,13 @@ bool editor_mode::update()
         draw_track_spread();
         draw_build_overlay();
         draw_track_intent();
-        speed_step();
+        m_speed.update();
         joints_step();
         draw_track_hints();
         draw_empty_scenery_hint();
         draw_profile_overlay();
         draw_infra_overlay();
-        draw_speed_overlay();
+        m_speed.draw_overlay();
         draw_joints_overlay();
     }
     else