Control play/pause buttons in slot of MainWindow via signal in Song class

The engine class as the component instance manager is the wrong place to
control the play/pause buttons. Instead emit a signal in the Song class
and update the buttons in a slot in MainWindow. This fixes problems with
GUI/pixmap operations happening outside the GUI thread when exporting a
project.

Closes #435.

include/MainWindow.h

@@ -107,6 +107,8 @@ public slots:
 	void togglePianoRollWin();
 	void toggleControllerRack();
 
+	void updatePlayPauseIcons();
+
 	void undo();
 	void redo();
 

include/engine.h

@@ -147,8 +147,6 @@ public:
 		return s_controllerRackView;
 	}
 
-	static void updatePlayPauseIcons();
-
 	static float framesPerTick()
 	{
 		return s_framesPerTick;

include/song.h

@@ -360,6 +360,7 @@ private:
 
 signals:
 	void projectLoaded();
+	void playbackStateChanged();
 	void lengthChanged( int _tacts );
 	void tempoChanged( bpm_t _new_bpm );
 	void timeSignatureChanged( int _old_ticks_per_tact,