gui: Make polling in ClientModel asynchronous

With this change polling runs in a different thread to prevent
disturbing the event loop.

src/qt/clientmodel.h

@@ -90,7 +90,8 @@ private:
     PeerTableModel *peerTableModel;
     BanTableModel *banTableModel;
 
-    QTimer *pollTimer;
+    //! A thread to interact with m_node asynchronously
+    QThread* const m_thread;
 
     void subscribeToCoreSignals();
     void unsubscribeFromCoreSignals();
@@ -110,7 +111,6 @@ Q_SIGNALS:
     void showProgress(const QString &title, int nProgress);
 
 public Q_SLOTS:
-    void updateTimer();
     void updateNumConnections(int numConnections);
     void updateNetworkActive(bool networkActive);
     void updateAlert();