Add option to reserve products for a sale.

This will allow the user to choose how many items he wants to set aside for
a sale. This way the item will be removed from the stock before the sale is
confirmed.

Change-Id: I3079af7ee1f68fe8556fde29bcfa503aeb7908fd
Fixes: http://bugs.async.com.br/show_bug.cgi?id=5574