cubicweb #3803629 Add some tools to make form section uicfg selection useful [in-progress]

Some more context and a dedicated predicate are needed to make useful usage of selectable form section uicfg.

An example use case is the edition of different relations of the same etype when edited with an inlined form in different etype forms (see test case).

done in<not specified>
load left0.000
closed by<not specified>
patch[forms,uicfg] Add the form itself as a context when selecting form-related uicfg objects. [in-progress][predicates] Add a predicate to be used with form-related uicfg objects to select them in the context of form or subforms editing a particular etype. [in-progress]