[FIX] purchase: fixed recomputation of unit price when changing product quantity or uom
[FIX] product: pricelist now consider the quantity in product uom qty for rule application purposes
[FIX] product_visible_discount: display full unit price only when the discount column is displayed

Without this, the unit price is visually set to full, the discount column is not visible, but the actual price used in computation is the discounted price, very misleading !