Fix xplane plugin header includes

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

refs #598

src/plugins/simulator/xplaneconfig/simulatorxplaneconfig.cpp

@@ -10,6 +10,8 @@
 #include "simulatorxplaneconfig.h"
 #include "simulatorxplaneconfigwindow.h"
 
+class QWidget;
+
 namespace BlackSimPlugin
 {
     namespace XPlane