Remove GUI related code from Song::stop

Add the new signal Song::stopped which is emitted when the song is
stopped. Connect the new slot method MainWindow::onSongStopped to that
signal. Remove all GUI updates from Song::stop and handle them in
MainWindow::onSongStopped.

include/MainWindow.h

@@ -252,7 +252,7 @@ private slots:
 	void onExportProject();
 	void onExportProjectTracks();
 	void onImportProject();
-
+	void onSongStopped();
 
 signals:
 	void periodicUpdate();

include/Song.h

@@ -428,7 +428,7 @@ signals:
 	void controllerAdded( Controller * );
 	void controllerRemoved( Controller * );
 	void updateSampleTracks();
-
+	void stopped();
 } ;