[Qt] rework setNumBlocks to have blockDate as parameter

- reduces some functional overhead and simplifies the code

src/qt/rpcconsole.h

@@ -63,8 +63,8 @@ public slots:
     void message(int category, const QString &message, bool html = false);
     /** Set number of connections shown in the UI */
     void setNumConnections(int count);
-    /** Set number of blocks shown in the UI */
-    void setNumBlocks(int count);
+    /** Set number of blocks and last block date shown in the UI */
+    void setNumBlocks(int count, const QDateTime& blockDate);
     /** Go forward or back in history */
     void browseHistory(int offset);
     /** Scroll console view to end */