Allow to override how concepts are rendered in ConceptScheme tab view

In SAEM we need to render the concept tree as a jquery tree. So we
introduce a render_tree method intended to be overidden by specific
cubes.

cubicweb_skos/views/__init__.py

@@ -153,6 +153,11 @@ class ConceptSchemeConceptsTab(tabs.TabsMixin, EntityView):
     __regid__ = 'skos_top_concepts_tab'  # don't use '.' in tab's regid
     __select__ = is_instance('ConceptScheme')
 
+    def render_tree(self, entity, children_rset):
+        treeid = 'skos_tree_%s' % entity.eid
+        self._cw.view('treeview', rset=children_rset, treeid=treeid,
+                      initial_thru_ajax=True, w=self.w)
+
     def entity_call(self, entity):
         rschema = self._cw.vreg.schema.rschema('in_scheme')
         if rschema.has_perm(self._cw, 'add', toeid=entity.eid):
@@ -164,8 +169,7 @@ class ConceptSchemeConceptsTab(tabs.TabsMixin, EntityView):
         if not rset:
             return
         if len(rset) < self._cw.property_value('navigation.page-size'):
-            treeid = 'skos_tree_%s' % entity.eid
-            self._cw.view('treeview', rset=rset, treeid=treeid, initial_thru_ajax=True, w=self.w)
+            self.render_tree(entity, rset)
         else:
             self._cw.view('paginated-list', rset=rset, w=self.w)