Automatically use all groups in a grouping

[mhughes2k]

Whilst it's really flexible to construct selected arbitrary groups, and selecting a grouping adds all of the groups, we've found that if a new group is added to a grouping, the next step to add that new group in is forgotten.

It would be really nice if it were possible to add a grouping and for it to ensure that all of the groups were presented even if they were added or removed.

It may be feasible to hook into \core\event\grouping_group_assigned and \core\event\grouping_group_unassigned events to do the same as adding / removing a grouping's groups via the form...