new do_action insertions for product inventory tab

I've added three do_actions which would make writing plugins for inventory management way easier, and give more flexibility for where custom fields appear. As is, if you want to add substantial fields to the inventory tab, you have to do it inside existing field areas, which is less than ideal for creating a whole new field. With the additional do_action statements, you can crete new fields after existing ones, rather than inside of them.