Move import functionality from Song to MainWindow

Move the functionality of the method Song::importProject into the
MainWindow as it mainly consists of GUI related actions.

Add a new method Song::setLoadOnLauch to ensure that the boolean
Song::m_loadOnLaunch is still set at the end of the import.

include/MainWindow.h

@@ -248,6 +248,7 @@ private slots:
 	void onToggleMetronome();
 	void onExportProject();
 	void onExportProjectTracks();
+	void onImportProject();
 
 
 signals:

include/Song.h

@@ -300,6 +300,8 @@ public:
 
 	void exportProjectMidi(QString const & exportFileName) const;
 
+	inline void setLoadOnLauch(bool value) { m_loadOnLaunch = value; }
+
 public slots:
 	void playSong();
 	void record();
@@ -309,8 +311,6 @@ public slots:
 	void togglePause();
 	void stop();
 
-	void importProject();
-
 	void startExport();
 	void stopExport();