Message center

- Addition of the module "message_center"
- Various fixes that shouldn't have been commited in this branch (sorry).
Deallocate is uncertain, because it may disappear from GMC messages when the childpool will be global (same children for all countries). Until we don't need it, we don't implement it.

For the depart method, we don't have enough information on the children right now to do it (state), plus we would need the user to be aware of procedure to do in this case. So it is a bit early to have it implemented, we should carefully separe functionnalities between GP and Odoo before.

Thanks for pointing this out, I will add the explanation in the comments.