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
We already have:
```
plugin.manager.menu.action
plugin.manager.menu.task
```
So ```plugin.manager.menu.link``` was the most consistent with those.