Refactor how warnings is used int the class Survey

[ukanga]

What is happening now?

Currently, it is passed around as a parameter in some class methods which does not look that clean especially where we depend on the object being updated.

What could make it better?

Use self.warnings.append("A warning message.") instead when added a warning or even self.add_warning("A warning message") with the function handling duplicates.

Implementation plan

It probably starts with a field in SurveyElement.

@@ -60,6 +60,7 @@ class SurveyElement(dict):
         "query": unicode,
         "autoplay": unicode,
         "flat": lambda: False,
+        "warnings": list
     }

     def _default(self):