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 @@ public Q_SLOTS:
 #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();

src/qt/utilitydialog.cpp

@@ -171,22 +171,20 @@ ShutdownWindow::ShutdownWindow(QWidget *parent, Qt::WindowFlags f):
     setLayout(layout);
 }
 
-void ShutdownWindow::showShutdownWindow(BitcoinGUI *window)
+QWidget *ShutdownWindow::showShutdownWindow(BitcoinGUI *window)
 {
     if (!window)
-        return;
+        return nullptr;
 
     // Show a simple window indicating shutdown status
     QWidget *shutdownWindow = new ShutdownWindow();
-    // We don't hold a direct pointer to the shutdown window after creation, so use
-    // Qt::WA_DeleteOnClose to make sure that the window will be deleted eventually.
-    shutdownWindow->setAttribute(Qt::WA_DeleteOnClose);
     shutdownWindow->setWindowTitle(window->windowTitle());
 
     // Center shutdown window at where main window was
     const QPoint global = window->mapToGlobal(window->rect().center());
     shutdownWindow->move(global.x() - shutdownWindow->width() / 2, global.y() - shutdownWindow->height() / 2);
     shutdownWindow->show();
+    return shutdownWindow;
 }
 
 void ShutdownWindow::closeEvent(QCloseEvent *event)

src/qt/utilitydialog.h

@@ -43,7 +43,7 @@ class ShutdownWindow : public QWidget
 
 public:
     ShutdownWindow(QWidget *parent=0, Qt::WindowFlags f=0);
-    static void showShutdownWindow(BitcoinGUI *window);
+    static QWidget *showShutdownWindow(BitcoinGUI *window);
 
 protected:
     void closeEvent(QCloseEvent *event);