Add preprocess hook to add layout link to Inline entity form

Problem/Motivation

If you are adding a node that is setup with mercury editor through an inline entity form it would be good if there was a link that took you directly to the mercury editor edit page for that entity.
If the primary method for adding content to an entity setup with mercury editor is through mercury editor itself, than it is presumably not feasible to edit that content inline, instead provide a link directly to the mercury editor page for that piece of content.

This hook is triggered when there is a table of entities in your inline entity form, it won't be triggered or added where there is only a single entity per inline entity form.

A future enhancement for this work would be to provide a configurable option to add this button to the inline entity form table, rather than have it be added automatically.

Proposed resolution

Apply the patch attached to this issue.