Clean category for product and website_sale module

Hello,

I have make following improvements

- Change string for categ_id field in product_template form.
- Move categ_id field into accounting tab.
- Move product_public_category object in website_sale module.
- Create pos_category object in Pointofsale module.
- Change model name in demodata in Pointofsale module.
- Create demodata for website_sale module.

Task link:
 https://www.odoo.com/web#id=7783&view_type=form&model=project.task&action=327

Pad link:
https://pad.odoo.com/p/r.fccQKXWsXYpcIMNk

Thanks,
ypa.