1) forbid deletion of main uoms & categories
2) warn modifications of main uoms data.
Except for the working Hours uom, which isn't a strict UoM
representation and could change between countries (and/or be deleted).
3) Also move the Dozen UoM to demo datas, it's a confusing UoM and isn't
used by the majority of databases.
4) Restrict deletion of used uoms on main models.
On sale orders and account moves the uoms couldn't be deleted thanks to the
SQL constraint covering the display_type logic, but the generic error message
is more clear for a user when deleting an uom.
Victor Feyens
ab76c421c2