refactor(editor): the parallel track is a tool of its own

The seventh tool out of editor_mode. It reads the selected set of tracks
and the tab shown through the services, digitizes the tracks it lays and
refreshes the plan after it sets one parallel to another. The panel shown
for two selected tracks is a public call of the tool, and the self-test
command parallel moves with it and lays the same tracks as before.

application/editormode.cpp

@@ -1450,7 +1450,7 @@ bool editor_mode::update()
             signal_move_update();
             draw_signal_overlay();
         }
-        draw_parallel_preview();
+        m_parallel.draw_overlay();
         if (m_track_tab == track_tab::lineside)
         {
             m_hekto.draw_overlay();