Add classes to Add Content links in Panels modal

Problem

In the file includes/add-content.inc the function template_preprocess_panels_add_content_link() preprocesses the add content links used in the modal. This preprocess adds the generic class panels-modal-add-config but does not add anything else specific to the "content type" being added.

Motivation

This makes it difficult for one to override the CSS of the Panels admin, which is necessary at times. For example, maybe you like the ability to add an "Existing node" or "New custom content" but you don't want to use the default buttons. If smart CSS classes were added based on the ctools content type subtype name, then you could do this.

Proposed solution

Attached is a patch that adds a class based on the $vars['content_type']['subtype_name'] variable.