[browser2] add a validate method in order to check objet before returning him

weboob/tools/browser2/page.py

@@ -497,6 +497,7 @@ class ItemElement(AbstractElement):
     _attrs = None
     klass = None
     condition = None
+    validate = None
 
     class Index(object):
         pass
@@ -528,6 +529,9 @@ class ItemElement(AbstractElement):
         except SkipItem:
             return
 
+        if self.validate is not None and not self.validate_object(self.obj):
+            return
+
         yield self.obj
 
     def handle_attr(self, key, func):