Move some window title updates from Song into MainWindow

Move the window title updates which are triggered due to a changed
modify state from Song to MainWindow.

Implementation details:
Add a new signal modified to Song and connect the new slot method
MainWindow::onSongModified to it.

Currently only the window title is updated in the slot. It is only
updated if the code is executed from the GUI main thread. This
implementation was taken over from the original implementation of
Song::setModified. The assumption here seems to be that the Song might
also be set as modified from other threads (which is a bad design).

Add a new private method Song::setModified(bool) and replace all direct
manipulations of m_modified in Song by calls to this method. This is done
to ensure that the signal can be emitted in all these cases. Make
Song::setModified() delegates to Song::setModified(bool) for the same
reason.

Other changes:
Slightly refactor MainWindow::resetWindowTitle to get rid of an
unnecessary if statement.

include/MainWindow.h

@@ -253,6 +253,7 @@ private slots:
 	void onExportProjectTracks();
 	void onImportProject();
 	void onSongStopped();
+	void onSongModified();
 
 signals:
 	void periodicUpdate();

include/Song.h

@@ -370,6 +370,8 @@ private:
 
 	void processAutomations(const TrackList& tracks, MidiTime timeStart, fpp_t frames);
 
+	void setModified(bool value);
+
 	AutomationTrack * m_globalAutomationTrack;
 
 	IntModel m_tempoModel;
@@ -429,6 +431,7 @@ signals:
 	void controllerRemoved( Controller * );
 	void updateSampleTracks();
 	void stopped();
+	void modified();
 } ;