Make DiscountWizard understand direct values

* Make DiscountWizard understand direct values for discount, that is, inform a
  value for the discount and it's percentage on the sale will be applied to the
  products.
* DiscountValue now has it's own module and is capable of setting the discount
  e's item editory itself, the editors calling it just have to update the objectlist.

Change-Id: Iacc66f787647e6f2353459b474a9683d1b6d5c51