[IMP/REF] Enabling inheritance of product.pricelist.items via context an...

...d additional methods without the need to overwrite method _price_get_multi() completely by adding two hooks

@odony
As I felt welcome for committing improvements to the core I did another valuable change as I needed it for a customer if I do not want to diverge from the core. Your review is welcome and of course merging soon is even more welcome.

I did not completely refactor but only added two hooks with the current algorithm to enable inheritance also for price_get which is the key for inheriting pricelist mechanism.