Include more migrations in default content migration

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 https://github.com/collective/collective.exportimport/pull/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.