[API proposal] modules printing to the course page

This is a proposal for a new (small) API for activity modules that want to print to the course page to display some information.

Examples of such modules are:

• label (included as an example in this patch)
• Face-to-face module (currently we tell people to hack Moodle to get this: http://docs.moodle.org/en/Integrating_Face-to-face_sessions_on_the_course_page

Basically, if a module has a function called modname_print_coursemodule_info, then it gets called instead of displaying the module name and icon on the course page.