Credit: original version done by Kirill Astashov of NetSpot (netspot.com.au),
finished and tweaked by sam.
This change adds conditional availability support for sections analagous to
that already available for activities. (Backend, UI, backup/restore.)
In order that this feature does not reduce performance, section cacheing has
also been added using a new course 'sectioncache' field analagous to modinfo.
The new feature integrates with activity availability so that activities
inside sections which are not available are automatically not available
themselves (meaning it works to restrict access).
The 'Show description' feature is about optionally including module description on the course page. This change include the necessary infrastructure - new database field, new FEATURE constant, changes to forms to display it, new language strings for it, backup and restore, and old course conversion. (I found the places to change by searching for an existing field in course_modules).
Documentation for the field is included in install.xml and in modinfolib.
This commit:
a) moves modinfo code into new library modinfolib.php
b) uses classes instead of stdClass objects, allowing a huge amount of documentation (and IDE completion)
c) adds hooks so that plugins other than forum can display messages like forum's 'unread', and plugins other than label can display html (apart from/as well as their view.php link) on the course view page
d) removes current hacks for forum and label (mainly in print_section but also across the code), replacing with new 'content' and similar variables [this is the reason for the changes in blocks, etc]
e) reduces size of modinfo in database (only when rebuilt) by excluding empty fields
The change is intended to be backward compatible and does not affect the format of modinfo in database.
