This patch proposes a major enhancement to assignement module, overloading existing assignment types with new group based types.

online : completed by onlinegrouped : all students of a group can edit the submission until it has been validated, Editing is arbitrated so that two simultaneous editing cannot accur across network. Grade and feedback is distributed among all group members

offline : as before, but marking is done based on group units, and grade and feedback is distributed among group members

upload multiple files : allow all student in a group to collectively add files to q single submission bucket until it will be validated by one member (any) of the group. Grades and feedback are distributed among group members

Stil to do : upload single (in progress)
Some tightening in access to some screens for ungrouped users
Some details (links to files) are not completely fixed.

Patch effect :
- adds a libgrouped.php
- patches sidewalks into lib.php functions, when the type is recognized as a grouped type
- adds type overloads for each user based types
- adds a locked field to assignment_submissions for arbitrating online editor access
- updates backuplib.php and restorelib.php consequently
- updates db/install.xml, db/upgrade.php consequently