Add family of callbacks to enable course editing form to be extended

There are a couple trackers for adding a concept of configurable course fields, similar to user profile fields. This is not another duplicate and is related but complementary. (MDL-34634, MDL-18319)

Instead this is a way for any plugin to programatically add fields which they require in order to work properly. At the moment you can create a new custom form and add it to the course admin menu, but this isn't where people expect to see course settings and by doing that way there is no ability to make fields required when creating or editing a course.

I see this working pretty well exactly the same as the coursemodule ones, ie:

xyz_course_edit_post_actions
xyz_course_standard_elements
xyz_course_validation