core: expose TrackingBrain.getExplanationValue

this way callers does not have to lookup explanation by catalog themselves

product/ERP5/bootstrap/erp5_core/ExtensionTemplateItem/portal_components/extension.erp5.InventoryBrain.py

@@ -288,6 +288,9 @@ class TrackingListBrain(InventoryListBrain):
   """
   List of aggregated movements
   """
+  def getExplanationValue(self):
+    return self._getObjectByUid(self.delivery_uid)
+
   def getDate(self):
     if not self.date:
       return