[repo] Add a publish/subscribe mechanism for inter-instance communication using zmq Each repo can have a publishing and any number of subscribing sockets whose addresses are specified in the instance's configuration. An application or cube can subscribe to some 'topics', and give a callback that gets called when a message matching that topic is received. As a proof of concept, this introduces a hook to clean up the caches associated with the repository when an entity is deleted. A subscription is added using Repository::zmq::add_subscription; the callback receives a list representing the received multi-part message as argument (the first element of the message is its topic).

action box
    Area visible in the upper left column. You have a list of available actions on the entity. The most frequently used entry is `modify`.

object
    All element created in the application
    Example: project, ticket, user, ...

relation editing module
    HTML widget that let you define new relations amongst objects.

relation
    It's a kind of 'smart' link between 2 objets of the application. It has a specific sense that determine dynamic behaviour and add a new logic of the content.