[ADD] product_unique_code.

@pedrobaeza the unique constraint doesn't apply to null values at all, at least it seems from my tests.
But I've tested leaving required=True with copy=False (which I didn't know about, thanks!) and it seems like it works: it creates and opens in edit mode the duplicate, but doesn't let you save without specifying a code (but it lets you discard, in which case the duplicated product will have null code o,o)

So, copy=False works with both required=True or required=False. What's your opinion on this? Should unique code imply required code? Also keep in mind that with required=True the code field is highlighted, which might hint the user he should change the field.