improve twinnable relations

Having a Behaviour Twinnable is no longer requirered as the context would be the from current page on front end.
A default one can be provided for back end purpose.

This API induces no breaking changes (a specif key must be used in the relation configuration). However, it could be clever to have this behaviour by default, even for twinnable models. Then the default value for context would be the one of the current model if it has one.

Note : as you can see with my last commit, I could not test has_one and has_many relation as I did not have these kind of relations in my current project. A review would be strongly advised and welcomed :)