Encapsulate context within course objects

Problem/Motivation

This was handled through hook_init() within each course object type provider module.

API changes

The context setting was moved into the course object, and invoked from parent in a more structured way:

• Each module that wishes to provide course context handlers must define a context callback in hook_course_handlers() (see @return documentation on that hook)
• The context handler callback is expected to return query parameters suitable for course_determine_context() (see @param documentation on that function)
• See examples in course extension modules, included in Course (example: Course book)