Create API for extending the Feeds UI.

This is a follow-up of #2305919: Return 404 when trying to edit a non-existent feed..

An empty page is shown when going to a non-existent subpage in the Feeds importer configuration, for example /admin/structure/feeds/node/non_existent. Ideally, a 404 page should be shown instead. However, we can't just generate a 404 for subpaths that Feeds does not know about, because this would possibly break contrib. For example, the modules Feeds Quick Mapping and Feeds Import Preview creates extra subpages.

Feeds should have an API for extending the Feeds UI. It would be nice if Drupal core's menu API could be used for that. See also the related issue #2333005: Change plugin path from plugin id to plugin type..