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/panner2d.cc

@@ -1013,7 +1013,7 @@ Panner2dWindow::set_width ()
 bool
 Panner2dWindow::on_key_press_event (GdkEventKey* event)
 {
-	return relay_key_press (event, &PublicEditor::instance());
+	return relay_key_press (event, 0);
 }
 
 bool