VAT number integrated in Ubercart core

No. I needed an extra field to hold the VAT registration number. There was a module that did that, but the number was not saved with the client's address. This patch adds a extra field just like telephone number field and stores the VAT number together with the client data. Since the roadmap is to integrate VAT handling, I thought this patch was something to contribute.

I use the field to store the obligatory tax reg. number for private clients. Spanish tax law prescribes this for legally correct invoices.

The description of uc_vat adds does not make any mention that it would allow to store a client VAT number with the billing information and see it back in order and invoice pages. That is the sole purpose of my patch. If uc_vat misses that, should I upload the patch there too? Doesn't make sense. It is not a module, it is a proposal to add one (optional, it can be deactivated like others) field to billing information. That way it is also restored together with billing info (unlike the VAT number module).