Add byte array API for extractor scripts

This enables converting QByteArrays to JS ArrayBuffers, and thus allows
custom binary barcode decoding also in extractor scripts.

The existing API related to byte arrays can be migrated from the barcode
API to the more appropriate place here, and more of that can be adapted
to use JS ArrayBuffers rather than opaque QByteArray variants.

First user is the MÁV barcode decoder.