[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 !
[FIX] corrected a word in a comment

closes #3423
[FIX] doc: fit code example to new API

When using this code as is, the 'ids' variable throws an exception for not
being defined. In the new API, 'ids' is no longer given as a function variable
but as a variable within 'self'.

close #3406