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
IMHO, this method doing more than one thing. So may be we can split some operations as separate protected/private methods (e.g. templete discovery). In other words, it may build after render instead build and render together.