cubicweb #2465490 Allow the display of related entities attributes in the attribute section of an entity [open]

In order to avoid the whole redefinition of the primary view of an entity, allow the display of some related entities in the attribute section of the primary entity. For that, enrich uicfg so as to be able to group attributes of several related entities.

My proposal : use something similar to primaryview_display_ctrl:

pv_ctrl = uicfg.primaryview_display_ctrl
pv_ctrl.tag_subject_of(('MySubbject', 'rtype', 'MyObject'),
            {'grouped_attrs': ['obj_attr1', 'obj_attr2']})

To go further, though not mandatory: define a 'ViewConfig' class that would ease the declaration of 'primaryview_section' and related uicfg helpers/

done in<not specified>
closed by<not specified>