Fix BlackGui header includes

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

refs #598

src/blackgui/components/corestatuscomponent.cpp

@@ -7,9 +7,11 @@
  * contained in the LICENSE file.
  */
 
-#include "corestatuscomponent.h"
+#include "blackgui/components/corestatuscomponent.h"
 #include "ui_corestatuscomponent.h"
 
+class QWidget;
+
 namespace BlackGui
 {
     namespace Components