Allow hidden values in groups

Tests are failing! No drag/drop support yet.
Fix “adds all question types and attributes” test

Changes to tests/static/all_question_types.xml are whitespace only (to improve diff output).
Fix remaining failing tests
Clean up tree 'into'/'inside' inconsistency
Use mug.options.isDataOnly where possible

Instead of comparing mug.__className to "DataBindOnly"
Implement drag+drop hidden values into group
Handle 'into' and 'inside' in move check
Refactor and simplify drag/drop/insert logic
Update references to hidden values on rename group
throw Error -> throw new Error
Fix type changing error criteria
Fix output ref update on move question
Merge pull request #262 from dimagi/repeat-hidden-values-v2

Allow hidden values anywhere in the tree