Make the barcode generator a move-only value type

Merged Volker Krause requested to merge vkrause/prison:work/vkrause/barcode-value-type into master

One possible improvement I can think of would be a convenience function for the common format() == Prison::Null checks, isNull() maybe?

The old API isn't removed yet, to allow porting existing KF6 consumers without causing immediate build breakage.

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