Add category icon for Schedule Publish

Problem/Motivation

The "Add field" function now displays icons representing the field types. As there is no icon defined for Scheduled Publish, it currently uses the default:

Steps to reproduce

Edit a content type, and select Add field.

Proposed resolution

Add an icon for Scheduled Publish.

Remaining tasks

Add icon.
Add library for icon.
Add hook_field_type_category_info_alter hook to tell Add Field to use icon.

User interface changes

Icon will be displayed on Add field page.

API changes

N/A

Data model changes

N/A