Bitcoin-Qt: cleanup / optimise addressbookpage

- don't show QR Code context menu, when USE_QRCODE=1 was not specified
  when compiling the client
- re-work on_showQRCode_clicked() for better readability and remove an
  unneeded duplicate check
- re-work on_signMessage_clicked() and on_verifyMessage_clicked() to match
  foreach in on_showQRCode_clicked(), which seems more robust / cleaner
- re-order context menu stuff to match real context menu layout
- add comments for all private slots in the class