Remove Song's dependency to MainWindow

Finally remove Song's dependency to MainWindow by moving the window
title update which is triggered due to a changed project file name from
the class Song into MainWindow.

Implementation details:
Add a new signal projectFileNameChanged to Song and connect the new slot
method MainWindow::onProjectFileNameChanged to it. Update the window
title whenever the slot is triggered.

Add a new private method Song::setProjectFileName which sets the project
file name and emits the signal (only if the file name really changes).
Call setProjectFileName everywhere where m_fileName was manipulated
directly. This is done to ensure that the signal can be potentially
emitted in all relevant situations.

Remove the calls to gui->mainWindow from
Song::createNewProjectFromTemplate.

These changes finally remove the include to "MainWindow.h". The include
for "ConfigManager.h" was previously done implicitly through the include
of "MainWindow.h" and therefore had to be added explicitly after the
latter is removed.

include/MainWindow.h

@@ -254,6 +254,7 @@ private slots:
 	void onImportProject();
 	void onSongStopped();
 	void onSongModified();
+	void onProjectFileNameChanged();
 
 signals:
 	void periodicUpdate();

include/Song.h

@@ -372,6 +372,8 @@ private:
 
 	void setModified(bool value);
 
+	void setProjectFileName(QString const & projectFileName);
+
 	AutomationTrack * m_globalAutomationTrack;
 
 	IntModel m_tempoModel;
@@ -432,6 +434,7 @@ signals:
 	void updateSampleTracks();
 	void stopped();
 	void modified();
+	void projectFileNameChanged();
 } ;