Stock landed costs security

This module turns the standard landed cost feature into its own application area inside Odoo, with a separate menu and its own permission group.

• A new "Landed Costs" menu appears with its own icon, opening the list of landed costs (stock.landed.cost).
• A new user group "User" is created under the Inventory category, so administrators can grant landed cost access to specific people.
• Users in that group get full rights on landed costs, their cost lines (stock.landed.cost.lines) and the valuation adjustment lines (stock.valuation.adjustment.lines), and read-only access to journals (account.journal) and journal entries (account.move).
• On the landed cost form, the Validate button is restricted to this new group or to Inventory managers.
• On vendor bills, the buttons to view or create landed costs, and the landed-cost related columns, are shown only to this group or to Inventory managers.

As a result, a user can handle the whole landed cost process without being given broad Inventory or Accounting permissions.