[Qt] rename rpcconsole window

- rework window title to not include Bitcoin - in front, as no other
  dialog does this
- favor a connect() call over an own function for clearing the traffic
  graph
- write monospace lowercase (seems to be correct after some web search)
  and add a comment that we should avoid / remove fixed font sizes

src/qt/rpcconsole.h

@@ -45,8 +45,6 @@ private slots:
     void on_sldGraphRange_valueChanged(int value);
     /** update traffic statistics */
     void updateTrafficStats(quint64 totalBytesIn, quint64 totalBytesOut);
-    /** clear traffic graph */
-    void on_btnClearTrafficGraph_clicked();
 
 public slots:
     void clear();
@@ -59,6 +57,7 @@ public slots:
     void browseHistory(int offset);
     /** Scroll console view to end */
     void scrollToEnd();
+
 signals:
     // For RPC command executor
     void stopExecutor();