Rework the menu system to be more entity reference based

Originally proposed here #1261130-24: [PP-1] Rewrite book module to use a standard, reusable API for creating hierarchies, I think this would make a good idea on its own.

One of the biggest issues I run into when training new team members is that they cannot oversee the menu system. There are menu links created from within entities (nodes, views, panels, etc) and there are manual links to custom urls. All my new team members follow the logical path, structure > menu > main menu > add menu item and they provide the url to their newly created view, while the older team members created the links from within the view. (This caused a lot of trouble with localization in Drupal 7.)

I would be very happy to see a method where the 'add menu item' would not only offer an url, but something like an entity reference as well (Since Drupal 8 it appears to be possible to reference nodes from the add menu item form). If node, views, panels, etc. would be able to tap into this system, it might even be possible to create and edit the same menu item from multiple places without issues?

So I'd like to propose the idea of having a centralized menu system, where the forms for nodes, views and menu links are the same and all these entities have a link to one single menu item entity (or more, why can't a node have 2 menu links?). Changing the entity in place updates it everywhere.

Postponed on #267855: Provide an entity hierarchy API