Barcode scanning with libzxing-cpp

Changes for 1.001

• re-work std::string handling to use the typemap
• fix "decoder" -> "decode" in the SYNOPSIS
• support all Imager image layouts
• require a recent enough ExtUtils::CppGuess and set the required C++ standard
• allow the zxing-cpp package name for pkg-config, which seems to be what packagers used before upstream decided on "zxing.pc". https://github.com/tonycoz/imager-zxing/issues/1