first compilable version of tabbable design.

I would have loved to split this apart, but there are just so many interrelated changes,
it makes little sense and would be a huge effort that would break future git bisect
use because so many intermediate commits would not compile

gtk2_ardour/step_entry.cc

@@ -504,7 +504,7 @@ StepEntry::on_key_press_event (GdkEventKey* ev)
                 }
 	}
 
-        return forward_key_press (ev);
+        return relay_key_press (ev);
 }
 
 bool