[Qt] ClientModel add method to get the height of the header chain

src/qt/clientmodel.h

@@ -51,6 +51,7 @@ public:
     //! Return number of connections, default is in- and outbound (total)
     int getNumConnections(unsigned int flags = CONNECTIONS_ALL) const;
     int getNumBlocks() const;
+    int getHeaderHeight() const;
 
     //! Return number of transactions in the mempool
     long getMempoolSize() const;