[FIX] product: add activity mixin on product

Product templates already use activities. However due to view inheritance
user can (try to) schedule activities on products. Those are not correctly
computed and displayed.

This commit somehow fixes that. It adds activity mixin on product itself
so that products have their own activities.

Due to inheritance mechanism activities on templates and product are
separated. This is a technical limitation and let us consider that as
a wow effect. Do you want a rainbowman ?

addons/product/models/product.py

@@ -78,7 +78,7 @@ class ProductProduct(models.Model):
     _name = "product.product"
     _description = "Product"
     _inherits = {'product.template': 'product_tmpl_id'}
-    _inherit = ['mail.thread']
+    _inherit = ['mail.thread', 'mail.activity.mixin']
     _order = 'default_code, name, id'
 
     price = fields.Float(