Include more migrations in default content migration

[pbauer]

Some export/imports that do not rely on other content (that might not yet exists at the time of importing) could be included in the default export/import of content. One example is constrains which is implemented like that in #71

Other options are:

• local roles
• discussions/comments
• portlets - I think portlets are safe to include because they can't hold relations or binary data, right?

We could add hooks like item = self.export_constrain(item, obj) and self.import_constrain(obj, item) for each to make it easy to override. We could also add checkboxes so enable/disable these extra-steps during export.

In client projects I also export/import some marker-interfaces and annotations but I don't think that could be generalized.
It would probably be enough to add some more dokumentation with simple examples how that do that.

[mauritsvanrees]

Portlets can contain relations, for example the Collection portlet points to a Collection. I guess this is with a relation.

[pbauer]

No, it only stores a uuid:

    widget(
        'uid',
        RelatedItemsFieldWidget,
        pattern_options={
            'selectableTypes': ['Collection']
        }
    )
    uid = schema.Choice(
        title=_(u"Target collection"),
        description=_(u"Find the collection which provides the items to list"),
        required=True,
        vocabulary='plone.app.vocabularies.Catalog',
    )