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
99% of the time discovery is not used at all by this class.

It's only used when rebuild is called.  So, why take the time to instantiate it in the constructor?