cubicweb #2406609 make ui configs selectables [done]

I would like to use uicfg tags to setup several automatic forms, that would be selectable according to the context. For instance, when the requesting user is admin I would you the default uicfg whereas a non-manager would use a custom one.

Care must be taken to keep backward compatibility.

Depends on

done in3.16.0
load left0.000
closed by#1dd655788ece make ui configurations selectable (closes #2406609)