Improved metronome (on/off during song, pattern and bb playback)

There is a new tool button that can be used to turn the metronome on and
off. Per default the metronome is turned off. When enabled the metronome
will during on song playback, pattern playback and BB playback. During
export it is ignored.

A new icon was added as well.

The state is currently stored in the Mixer. It might make sense to put
the metronome configuration in its own class in the future. The state is
currently not stored in the file but this might be a good choice for now
until a better place is found for the metronome data.

Also removed some repeated calls to Engine::getSong() and
Engine::fxMixer().

include/MainWindow.h

@@ -185,6 +185,8 @@ private:
 
 	QMenu * m_viewMenu;
 
+	ToolButton * m_metronomeToggle;
+
 private slots:
 	void browseHelp();
 	void fillTemplatesMenu();
@@ -193,6 +195,7 @@ private slots:
 	void updateRecentlyOpenedProjectsMenu();
 	void updateViewMenu( void );
 	void updateConfig( QAction * _who );
+	void onToggleMetronome();
 
 
 	void autoSave();

include/Mixer.h

@@ -355,6 +355,9 @@ public:
 
 	void changeQuality( const struct qualitySettings & _qs );
 
+	inline bool isMetronomeActive() const { return m_metronomeActive; }
+	inline void setMetronomeActive(bool value = true) { m_metronomeActive = value; }
+
 
 signals:
 	void qualitySettingsChanged();
@@ -457,6 +460,8 @@ private:
 
 	MixerProfiler m_profiler;
 
+	bool m_metronomeActive;
+
 	friend class Engine;
 	friend class MixerWorkerThread;