Provide a way to extract a menu plugin from a menu form

Problem/Motivation

For modules like http://drupal.org/project/menu_attributes you need a way to get a plugin ID/plugin from a plugin menu form, so you can update its values.

This allows to write generic submit functions on menu forms, which updates the values of the menu link plugins.

Proposed resolution

• Add a menu like getMenuLinkInstance to \Drupal\Core\Menu\Form\MenuLinkFormInterface
• If this isn't possible, add an additional interface, and implement it throughout core?

Remaining tasks

User interface changes

API changes

Data model changes