[Qt] Add Smartfee to GUI

src/qt/bitcoinamountfield.h

@@ -48,6 +48,9 @@ public:
     /** Make field empty and ready for new input. */
     void clear();
 
+    /** Enable/Disable. */
+    void setEnabled(bool fEnabled);
+
     /** Qt messes up the tab chain by default in some cases (issue https://bugreports.qt-project.org/browse/QTBUG-10907),
         in these cases we have to set it up manually.
     */