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

Hmm, I don't care too much about the naming, it's just slightly annoying to find/replace.

I might go simply for menu.tree if changing it.  However, I don't think the service name is that important.