Reorder/split lib.php functions

[jcharaoui]

Right now lib.php is a mess, functions should have some kind of order instead of appearing anywhere. We have :

• Database functions
• Term functions
• (un)Enrolment functions
• Query functions (ie. get enrolments)

Furthermore, not all of these functions could/should be overridable, such as 'get_enrolments'. We should add overridability on a need-only basis, I think.