[Qt] Replace status bar unit icon with actual images

src/qt/bitcoinunits.h

@@ -36,6 +36,8 @@ public:
     static QList<Unit> availableUnits();
     //! Is unit ID valid?
     static bool valid(int unit);
+    //! Identifier, e.g. for image names
+    static QString id(int unit);
     //! Short name
     static QString name(int unit);
     //! Longer description