fix incorrect sensitivity of automation acts at session load

Session skipped EditingContext::set_sensitive() by calling too far back up its
inheritance tree.

gtk2_ardour/editor.cc

@@ -1198,7 +1198,7 @@ Editor::update_title ()
 void
 Editor::set_session (Session *t)
 {
-	SessionHandlePtr::set_session (t);
+	EditingContext::set_session (t);
 
 	section_marker_bar->clear (true);