Qt: Add GUI feedback and control of network activity state.

Add getNetworkActive()/setNetworkActive() method to client model.
Send network active status through NotifyNetworkActiveChanged.
Indicate in tool tip of gui status bar network indicator whether network activity is disabled.
Indicate in debug window whether network activity is disabled and add button to allow user to toggle network activity state.

src/qt/clientmodel.h

@@ -67,6 +67,10 @@ public:
     bool inInitialBlockDownload() const;
     //! Return true if core is importing blocks
     enum BlockSource getBlockSource() const;
+    //! Return true if network activity in core is enabled
+    bool getNetworkActive() const;
+    //! Toggle network activity state in core
+    void setNetworkActive(bool active);
     //! Return warnings to be displayed in status bar
     QString getStatusBarWarnings() const;
 
@@ -90,6 +94,7 @@ Q_SIGNALS:
     void numConnectionsChanged(int count);
     void numBlocksChanged(int count, const QDateTime& blockDate, double nVerificationProgress, bool header);
     void mempoolSizeChanged(long count, size_t mempoolSizeInBytes);
+    void networkActiveChanged(bool networkActive);
     void alertsChanged(const QString &warnings);
     void bytesChanged(quint64 totalBytesIn, quint64 totalBytesOut);
 
@@ -102,6 +107,7 @@ Q_SIGNALS:
 public Q_SLOTS:
     void updateTimer();
     void updateNumConnections(int numConnections);
+    void updateNetworkActive(bool networkActive);
     void updateAlert();
     void updateBanlist();
 };