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.h

@@ -111,6 +111,11 @@ private:
     /** Enable or disable all wallet-related actions */
     void setWalletActionsEnabled(bool enabled);
 
+    /** Connect core signals to GUI client */
+    void subscribeToCoreSignals();
+    /** Disconnect core signals from GUI client */
+    void unsubscribeFromCoreSignals();
+
 signals:
     /** Signal raised when a URI was entered or dragged to the GUI */
     void receivedURI(const QString &uri);