This diff adds an option to enable the QR code display (in the payment
request dialog).
The QR code is enabled by default and can be disabled by setting the
By default the QR code is en`-DENABLE_QRCODE` variabled if on the libqrencode library is found
and disabled otherwise.cmake command line.
If either QT or the wallet is not built, It can be forced enabled or disabled by setting
the `-DENABLE_QRCODE` on the cmake command linethe option is not used.