Relationship plugin data:

        'title' => The title to display.
        'description' => Description to display.
        'keyword' => Default keyword for the context created by this relationship.
        'required context' => One or more ctools_context_required/optional objects
          describing the context input.
          new panels_required_context(t('Node'), 'node'),
        'context' => The callback to create the context. Params: ($context = NULL, $conf)
        'settings form' => Settings form. Params: $conf
        'settings form validate' => Validate.