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

@@ -245,6 +245,7 @@ private:
 #endif
     int returnValue;
     const PlatformStyle *platformStyle;
+    std::unique_ptr<QWidget> shutdownWindow;
 
     void startThread();
 };
@@ -411,7 +412,7 @@ 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::showShutdownWindow(window);
+    shutdownWindow.reset(ShutdownWindow::showShutdownWindow(window));
 
     qDebug() << __func__ << ": Requesting shutdown";
     startThread();