Add proper exit to shutdown code

The old shutdown code did not correctly exit but instead just
continuously called the QApplication notify function until it
SEGFAULTed. This led to crashes whenever SteamVR was shut down. Users
with Windows Error Reporting turned on would get a 100+ MB crashdump
file along with increased shutdown time for SteamVR.

For clarity the shutdown functions should probably be collected in a
single function.

src/overlaycontroller.cpp

@@ -671,9 +671,11 @@ void OverlayController::mainEventLoop()
             m_chaperoneTabController.shutdown();
             Shutdown();
             QApplication::exit();
-            return;
+
+            LOG( INFO ) << "All systems exited.";
+            exit( EXIT_SUCCESS );
+            // Does not fallthrough
         }
-        break;
 
         case vr::VREvent_DashboardActivated:
         {