plugins/barcode: Call it a "QR code" in the UI

This plugin generates a QR code, but refers to it as a barcode in the UI strings. In English at least, this is confusing; barcodes and QR codes are considered different things, rather than one being a subset of the other. So let's change the base English text to call the generated image a QR code rather than a barcode. Any other language where the translation for "barcode" is appropriate can substitute that in the translated text.

cc @teams/vdg