Move GUI dependencies out of Song::exportProjectMidi

Move all GUI dependencies out of Song::exportProjectMidi and move them
into the MainWindow.

Show the GUI in the new slot method MainWindow::onExportProjectMidi and
delegate to Song::exportProjectMidi once the file name has been
determined. The file name is given as a parameter to
Song::exportProjectMidi which still contains the business logic of
exporting the data.

include/MainWindow.h

@@ -175,6 +175,9 @@ public slots:
 
 	void autoSave();
 
+private slots:
+	void onExportProjectMidi();
+
 protected:
 	virtual void closeEvent( QCloseEvent * _ce );
 	virtual void focusOutEvent( QFocusEvent * _fe );

include/Song.h

@@ -298,6 +298,7 @@ public:
 		return m_timeSigModel;
 	}
 
+	void exportProjectMidi(QString const & exportFileName) const;
 
 public slots:
 	void playSong();
@@ -311,7 +312,6 @@ public slots:
 	void importProject();
 	void exportProject( bool multiExport = false );
 	void exportProjectTracks();
-	void exportProjectMidi();
 
 	void startExport();
 	void stopExport();