qt: Avoid shutdownwindow-related memory leak

Store a reference to the shutdown window on BitcoinApplication,
so that it will be deleted when exiting the main loop.

src/qt/bitcoin.cpp

@@ -246,6 +246,7 @@ private:
 #endif
     int returnValue;
     const PlatformStyle *platformStyle;
+    std::unique_ptr<QWidget> shutdownWindow;
 
     void startThread();
 };
@@ -410,6 +411,11 @@ void BitcoinApplication::requestInitialize(Config &config)
 
 void BitcoinApplication::requestShutdown()
 {
+    // Show a simple window indicating shutdown status
+    // Do this first as some of the steps may take some time below,
+    // for example the RPC console may still be executing a command.
+    shutdownWindow.reset(ShutdownWindow::showShutdownWindow(window));
+
     qDebug() << __func__ << ": Requesting shutdown";
     startThread();
     window->hide();