Sale product set variant

This module extends the Sale Product Set feature so that a set can be defined at the level of a product template and its variants, instead of only a single product.

• On a product set, each line now shows a Product Template and a Variant field. The old single-product field is hidden. You pick the template, then choose one or more variants of it.
• When you use the "Add set" wizard on a sale order, the wizard shows the set lines with template, variants and quantity. If a template has only one variant, it is filled in automatically; if it has several, you must choose the variants yourself.
• If you try to add a set line whose template has several variants but none selected, Odoo stops you with an error asking you to select the appropriate variants.
• For each selected variant, a separate sale order line is created, with the quantity multiplied by the wizard quantity, the variant's unit of measure, and the line sequence taken from the set line.
• The module touches product.set.line, product.set.add and a new wizard line model product.set.add.line, and adds fields to the product set form and the add-set wizard.