Implement menu links as plugins, including static admin links and views, and custom links with menu_link_content entity, all managed via menu_ui module

This PR is just for interactive review since it's such a big changeset  - please summarize comments at: https://drupal.org/node/2256521
This is what we have already in the local action and local task managers.

Since it's totally internal, we could refine it later, but it's the same as other classes in core are now.