This commit simply move code for both rating.mixin
and rating.parent.mixin in their own file to ease
the reading of code, and fit the guidelines.
Task-1903565
rating module provide now post a message with a
rating value, though `message_post` method.
'mail.message' also has now a `rating_value` field
to ease searching and filtering.
This will help the ecommerce (and other website modules)
to post a comment associate with a rating.
jem-odoo