cubicweb #2818471 selectable uicfg and "inheritance" [open]

As of today, if one defines uicfg rules in the base/default uicfg for some rdefs, and then create a specialized ruleset, one does not benefit from the initial work. We start again from a completely void ruleset.

It might prove convenient to (by default/by explicit switch) enable getting back some of the base rules in a non-base ruleset.

e.g. the Repository entity in the vcsfile cube has some rules governing the primary view and the edition form. When writing a specialized ruleset to handle some corner case (e.g. value of an app-defined rtype), it's not too pleasant to be forced to write the full base ruleset again.

Maybe, in the presence of an inheritance chain, we want to copy the parent rules.

I suspect this is a bug.

done in<not specified>
load left1.000
closed by<not specified>