Auto Select default engine

I ran into a problem where I installed this module and the default engine was set to "qr_codes_google_chart:google_chart", but I only enabled the "qr_codes_qrencode" module. On the QR Codes->Global Setting screen, it showed that libqrencode was selected, but really it was not (and I never clicked save, because everything looked good). The qr codes images were not being generated, and there was no error message anywhere saying that the selected engine wasn't enabled.

So, attached is a patch that auto-select the first available engine if the currently selected one is not enabled.