qt: Show window while shutting down

Makes it clear to the user that the application is still wrapping up
and the computer should not be turned off until it is finished.

src/qt/bitcoin.cpp

@@ -32,6 +32,8 @@
 #include <QTranslator>
 #include <QWeakPointer>
 #include <QThread>
+#include <QVBoxLayout>
+#include <QLabel>
 
 #if defined(QT_STATICPLUGIN)
 #include <QtPlugin>
@@ -347,17 +349,34 @@ void BitcoinApplication::requestShutdown()
     window->setClientModel(0);
     window->removeAllWallets();
     guiref.clear();
+
     delete walletModel;
+
+    // Show a simple window indicating shutdown status
+    QWidget *shutdownWindow = new QWidget();
+    QVBoxLayout *layout = new QVBoxLayout();
+    layout->addWidget(new QLabel(
+        tr("Bitcoin Core is shutting down...\n") +
+        tr("Do not shut down the computer until this window disappears.")));
+    shutdownWindow->setLayout(layout);
+
+    // 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();
+
+    // Request shutdown from core thread
     emit requestedShutdown();
 }
 
 void BitcoinApplication::initializeResult(int retval)
 {
     LogPrintf("Initialization result: %i\n", retval);
-    /// Set exit result: 0 if successful, 1 if failure
+    // Set exit result: 0 if successful, 1 if failure
     returnValue = retval ? 0 : 1;
     if(retval)
     {
+        // Miscellaneous initialization after core is initialized
         optionsModel->Upgrade(); // Must be done after AppInit2
 
         PaymentServer::LoadRootCAs();