get editor controls layout/midi track header/scroomer to be the correct height

Using is_mapped() is wrong, since if the editor was not visible (i.e. program has not yet shown the editor
"tab", then _group_tabs will have is_visible() == true but is_mapped() == false

gtk2_ardour/editor_canvas.cc

@@ -313,7 +313,7 @@ Editor::reset_controls_layout_width ()
 	edit_controls_vbox.size_request (req);
 	w = req.width;
 
-        if (_group_tabs->is_mapped()) {
+        if (_group_tabs->is_visible()) {
 		_group_tabs->size_request (req);
                 w += req.width;
         }