Allow to specify permission dependencies

[Cellule]

Allow to specify dependencies for certain permissions.

For instance bill creation need to be able to see the inventory.
Prevent to set this permission until dependencies are resolved (show a message why this is unavailable with a list of permissions)

[mathieumg]

Currently we would have no simple way of expressing these dependencies, and as discussed the underlying problem could be avoided with #277.

[Cellule]

For now I would see no problem at all using the key __dependencies