Use enum for barcode types
Barcode type is currently given as a string, directly from zbar. For convenience for Aperture users, this should be converted to an enum type. That way, it's clearer what the possible options are, and it can be used in switch statements or using == rather than g_strcmp().
Also, instead of a BarcodeResult type, the barcode-detected signal should simply have two arguments: barcode type and data.