[IMP] product: only template child / multiple template childs form view improvements

[IMP] product: only template child / multiple template childs form view improvements
- added a is_only_child field on product.product, telling whether the
product is the only child of the product.template
- improved the product form view, to set readonly all fields coming fgrom
the template when the product is not the only child. This way, the user
is more aware that changing a value in the template changes the value
for all child products. He will have to do those changes on the template
form. A link to the template has been added in the product form view.

bzr revid: tde@openerp.com-20140122095826-8k0wl7af57iiyify