add ref=* as real field to the vending machine presets

[Elefant-aus-Wuppertal]

Hello,
In my country many vending machines (e. g. those vending sweets, public transport tickets, cigarettes) have a special number given by the operator to identify every single machine, so I think that's definetely a ref. Even though iD renders given refs still, might it not be a good idea to add the ref=*-field to the vending machine presets so that users can enter the reference numbers there?