qt: Remove global references in bitcoin.cpp

Remove the need for global references `guiref` and
`splashref` by making the BitcoinGUI and SplashScreen
classes register for the UI interface signals themselves.

src/qt/bitcoingui.cpp

@@ -170,10 +170,16 @@ BitcoinGUI::BitcoinGUI(bool fIsTestnet, QWidget *parent) :
 
     // Initially wallet actions should be disabled
     setWalletActionsEnabled(false);
+
+    // Subscribe to notifications from core
+    subscribeToCoreSignals();
 }
 
 BitcoinGUI::~BitcoinGUI()
 {
+    // Unsubscribe from notifications from core
+    unsubscribeFromCoreSignals();
+
     GUIUtil::saveWindowGeometry("nWindow", this);
     if(trayIcon) // Hide tray icon, as deleting will let it linger until quit (on Ubuntu)
         trayIcon->hide();
@@ -851,3 +857,29 @@ void BitcoinGUI::detectShutdown()
     if (ShutdownRequested())
         QMetaObject::invokeMethod(QCoreApplication::instance(), "quit", Qt::QueuedConnection);
 }
+
+static bool ThreadSafeMessageBox(BitcoinGUI *gui, const std::string& message, const std::string& caption, unsigned int style)
+{
+    bool modal = (style & CClientUIInterface::MODAL);
+    bool ret = false;
+    // In case of modal message, use blocking connection to wait for user to click a button
+    QMetaObject::invokeMethod(gui, "message",
+                               modal ? GUIUtil::blockingGUIThreadConnection() : Qt::QueuedConnection,
+                               Q_ARG(QString, QString::fromStdString(caption)),
+                               Q_ARG(QString, QString::fromStdString(message)),
+                               Q_ARG(unsigned int, style),
+                               Q_ARG(bool*, &ret));
+    return ret;
+}
+
+void BitcoinGUI::subscribeToCoreSignals()
+{
+    // Connect signals to client
+    uiInterface.ThreadSafeMessageBox.connect(boost::bind(ThreadSafeMessageBox, this, _1, _2, _3));
+}
+
+void BitcoinGUI::unsubscribeFromCoreSignals()
+{
+    // Disconnect signals from client
+    uiInterface.ThreadSafeMessageBox.disconnect(boost::bind(ThreadSafeMessageBox, this, _1, _2, _3));
+}