Fix BlackGui header includes

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

refs #598

src/blackgui/pluginconfigwindow.h

@@ -13,8 +13,12 @@
 #define BLACKGUI_PLUGINCONFIGWINDOW_H
 
 #include "blackgui/blackguiexport.h"
+
+#include <QObject>
 #include <QWidget>
 
+class QShowEvent;
+
 namespace BlackGui
 {
     /**