Qt: allow "emergency" shutdown during startup

- allows closing our splash screen to abort startup

src/qt/splashscreen.cpp

@@ -177,5 +177,6 @@ void SplashScreen::paintEvent(QPaintEvent *event)
 
 void SplashScreen::closeEvent(QCloseEvent *event)
 {
+    StartShutdown(); // allows an "emergency" shutdown during startup
     event->ignore();
 }