VT72x: Support a UK Date Separator Format

Traditionally vtiger has only used a "-" as the separator between parts of a date value when shown to a user.

A recent commit (e5506287) adds support to use a "." instead which is common in parts of Europe.

In the UK, and possibly other countries too, the date is almost always displayed using a "/" as the separator, i.e. 28/02/2020

It would be nice to be able to support this as well as the dot separator...