Bitcoin-Qt: better copyright year handling in AboutDialog

- this change allows us to keep the translation without the need to
  re-translate any string, when we update the copyright year
- copyright symbol is changed to HTML to ensure we get no encoding
  issues and it's removed from the translation string so translators don't
  break it by mistake

src/qt/aboutdialog.h

@@ -18,6 +18,7 @@ public:
     ~AboutDialog();
 
     void setModel(ClientModel *model);
+
 private:
     Ui::AboutDialog *ui;