Added possibility to define facet/dialog specific policies
    
    After deleting section as a special type of object a new way of defining inter-field logic is needed.
    
    For this purpose a facet_policy was created. It is a simple object with init() method. Init method should contain logic for attaching to fields' or widgets' events.
    When a policy is added to facet or dialog its container property should be set to that facet or dialog. It gives the policy an access to fields and widgets.
    Init method should be called after widgets creation.
    
    https://fedorahosted.org/freeipa/ticket/2040