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
same as above. this sounds like "there is a link tree in menu" instead of "menu link tree". for me menu_link_tree sounds good.