[IMP] point_of_sale, product: change model 'pos.category' of the pos into product.public.category in product. product.public.category is used for eg by pos and website_sale. Add some demo data for this category. Add a domain in pos 'model.js' to select non empty categories

bzr revid: chm@openerp.com-20130828151658-3isgyl93c3hwp1pc