Fixed Recorder and Session to allow transfer of recorders when changing

session.

Mixer.cpp

@@ -672,6 +672,9 @@ void Mixer::swap()
     session_ = back_session_;
     back_session_ = tmp;
 
+    // swap recorders
+    back_session_->transferRecorders(session_);
+
     // attach new session's nodes to views
     for (auto source_iter = session_->begin(); source_iter != session_->end(); source_iter++)
     {