grouping of constraints
Current: Double click on a project. All constraints available appear when building a configuration, without grouping.
Desired: The constraints are grouped per type of project (in a not fixed manner, just as a default organisation). More generally, the user should be able to create his/her own folders to organise constraints at will.
(from redmine: issue id 3038, created on 2017-08-18, closed on 2018-05-29)