cubicweb #2731165 rtype immutability support [deprecated]

Having immutable rtypes can be extremely useful.

The goal is to provide a generic implementation of rtype immutability.

This should be (later) further extended with yams annotation support to alleviate the need to annotate the orm entities to specify which rtypes ought to be immutable.

* declaration of rtype immutability (per rtype)

* automatic uicfg thing that hides immutable fields on edition forms

* hooks that prevent immutable rtypes edition

Syntactic details, API, implementation will be better explained through the patch and provided tests.

done in3.18.5
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