Fix default Brightness value/behavior adds shutdown button (#412)

* added exit button

* fixed visible state

* added shutdown button fixed default brightness issues

* fixed compiler warning

src/overlaycontroller.h

@@ -184,6 +184,7 @@ public:
     virtual ~OverlayController();
 
     void Shutdown();
+    Q_INVOKABLE void exitApp();
 
     bool isDashboardVisible()
     {