[FIX] product: find products of subcategories

When searching products for a specific category, display the products of the whole category subtree.
[FIX] website_mail: avoid error when message.body is empty
[FIX] point_of_sale: account selection for payments

Registered payment uses the partner receivable account. As this field is
a property field, it will select different accounts based on the user that
registers the payment (in multicompany).
Should use the company of selected journal instead of the one of the user.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64