Add "About Qt" menu option to show built-in Qt About dialog

- Most Qt programs do this, and it can be useful to find out what version of Qt was built against.

src/qt/bitcoingui.h

@@ -82,6 +82,7 @@ private:
     QAction *exportAction;
     QAction *encryptWalletAction;
     QAction *changePassphraseAction;
+    QAction *aboutQtAction;
 
     QSystemTrayIcon *trayIcon;
     Notificator *notificator;