Fix bug that prevented loading some forms with duplicate question IDs

Forms with duplicate question IDs of different question types may still fail.
Minor fixes, remove unused code
Apply change from PR #227
Prevent adding duplicate questions
Refactor mug property change handler

Split handler so change can be prevented.
Minor refactoring/cleanup
Fix obscure bug in renaming a question

The form must have a ReadOnly question as a child of the question being renamed.
Merge pull request #228 from dimagi/127310-duplicate-questions

Fix error handling for duplicate question ID