[ADD] sale_coupon: introduce coupon mechanism on sales order

Introduces 2 new concepts:

Promotion programs:
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Using promotion programs allows you to propose to your customers:
- Promotional offers limited in time or a limited order number.
- Public code printed in advertisements, magasines, etc.
- Specific offers based on a customers group or a product set.
- A combination of the preceeding points.

You may choose to apply the program reward immediately on the sales order or to propose the reward for a next order. In the second case a coupon will be generated at the order confirmation for the customer to use it later.

Coupon programs:
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Using coupon programs allows you to propose to your customers:
- Free products or discounts if some conditions on products or purchased amount are met.
- Promotional offers limited in time.
- Generate coupons for a customers group.
- Specific offers based on a customers group or a product set.
- A combination of the preceeding points.

A coupon is applied by entering a valid code if the program rules are met and if the reward is already in the order lines.

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