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
not sure why the discovery(would be a heavy lift) is part of first call instead of object creation time (i.e. __construct)