Add support for hierarchies and unselectable items in AbstractSelect2Choice

[cwellekoetter]

The AbstractSelect2Choice component supports simple choices of elements, but no hierarchical structure like optgroups and options.

This would be a nice feature.

[Johannes-Esseling]

+1

Is somebody working on this feature? Because it could be really useful for me. The problem I face right now is that to implement it myself I would need to rewrite the onResourceRequested()) in the AbstractSelect2Choice which is quite the challenge.