Fix BlackGui header includes

* Include only what is used
* Use forward declaration when possible
* Sorted includes

refs #598

src/blackgui/levelmeter.h

@@ -15,9 +15,15 @@
 #define BLACKGUI_LEVELMETER_H
 
 #include "blackgui/blackguiexport.h"
+
+#include <QColor>
+#include <QObject>
 #include <QTime>
 #include <QWidget>
 
+class QPaintEvent;
+class QTimer;
+
 namespace BlackGui
 {