Identify Questionmark-specific barcodes by implementing `#questionmark?`

[Narnach]

Questionmark's internal systems currently check if a barcode is prefixed with 9669 to see if it's a QM-specific dummy barcode or if it's an official GTIN.

It's cleaner to have all parsed GTINs respond to #questionmark? and have the default ones return false while "ours" return true.

[wvengen]

Ah yes, that's a great idea!