[IMP] account,sale,...: Remove 'internal category' menuitems when not useful

Purpose
=======

Internal categories are only useful for accounting & inventory.
-> so they should be only visible for such modules

Also terms are confusing: internal category -> category  "internal" is not obvious

Specification
=============

- remove internal categories menus from:
    sales
    website_sale
    pos

- rename menu in:
    purchase
    inventory

- add menu to:
    accounting
    products must be visible even if account.accountant is not installed
    categories only visible if account.accountant is installed

- rename categories in product form:
    - category
    - pos category