For performance, dont need to get the entity type, we know for sure what it will be.
For the same reason isDeletable() returns getDeleteRoute, do the same for isTranslatable.
Clean up on MenuLinkInterface and a bit on MenuLinkManagerInterface.
Wrapped to 80 chars, some comments in MenuLinkInterface.
fix interface doxygen
fully populate the doxygen for createInstance() since we can't mixed with @inheritdoc
fixes for menu link content interface
START PART 2:  https://www.drupal.org/node/2301273
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkTree-review-do-not-test.patch