Name: Add a display model

Abstract: How to Provide more display models for article and derived content
  types

Preconditions:
 - the user has write access to the Zope instance Product directory
 - the user knows the Python programming language
 - the user knows the internals of Zope and Plone

Trigger: the user is a developper that wants to add a new model.

Main course:
 1. Create an interface for this new view (i.e, "IExampleModel")
 2. Create a class that inherits from 
    PloneArticle.browser.article.PloneArticleModelView and implements the 
    interface previously created (Zope 3/Five "implements").
    This class must the following attributes:
     - template_id: the template name (i.e. example_model_view)
     - title: unicode english title
     - title_msgid: for translation
     - class documentation is used for description. Fill it! (ascii/english)
     - description_msgid: for translating the description
     - icon: the name of the image to use in the "display" tab for this template
 3. Register this class with PloneArticle.model.registerArticleModel
 4. Declare the view with ZCML (adapt it):
    <browser:page
      for=".interface.IPloneArticle"
      name="example__model_view"
      class=".example.module.ExampleModel"
      permission="zope.Public"
      allowed_interface=".browser.interface.IPloneArticleModelView
                         .interface.IExampleModel"
      />
 5. Provide the template and icon in your skins directory
 6. Translations of title and description are searched in the domain
    "plonearticle-model", so you need an po file in this domain with entries for
    title_msgid and description_msgid
 7. Restart Zope
 8. In a plone instance, go to Site setup -> PloneArticle setup. In "models 
    management" tab, add the model to the list of selected models.
 9. The model is available in the "display" action and in the form of the 
    "display tab"

Comments:
  PloneArticles models are all defined in PloneArticle/browser,
  PloneArticle/browser.zcml; their templates and icons reside in
  PloneArticle/skins/pa_models
