Rationalize context usage in client
The construction of the context inside the client is not rationalized. Sometimes, we add the rpc.CONTEXT, we use the screen static context (in sao) or we update multiple times the parent context etc.
I think we should rationalize by always use the context of the group as reference and to always add the rpc context.
Also the expr_eval uses the context as base which is mixed with the fields. Instead we should enforce to always use explicitly the 'context' keyword. And the '_user' key is added which does not seem to be used anywhere, so it could be removed. If someone need it, he could add to the user context.