Handle easily multiple variants on Purchase Orders

This module adds a button on the purchase order form, "Add or Modify Variants", visible only in draft or sent state and only for users who can manage product variants. It opens a pop-up window where you pick a product template and see a grid of all its variants, with one axis for the first attribute and another for the second attribute.

In the grid you type the quantity you want for each combination. When you press "Transfer to order", the module updates the purchase order lines: quantities are changed, new lines are created for variants that had no line, and lines whose quantity you set to zero are removed. Only templates with one or two attributes that create variants are handled; templates with more attributes are ignored.

Each purchase order line also gets a small "Modify Variants" button that opens the same grid pre-filled with that product's template, so you can adjust variants directly from the line. The button only appears for draft or sent orders and for lines whose product has attribute values.

The module works on purchase.order.line and adds a new wizard model purchase.manage.variant with its line model purchase.manage.variant.line. It depends on the 2D matrix widget to display the grid.