Added a debugging check to enforce this requirement.
- Added controller statistics.

- Wrap message in FailableView.
- Name Statistics job correctly.

- Fix jobs into their own files.
- Minor changes.

- All observers now unregister properly at the controller and model.
- Some other minor changes.

- Fix crash due to statistics module.

- symbols in internal (Terminals, Non-Terminals) now have a link back to the AST.

if any required jobs fail.

- Add a FailableView base class that allows views to replace their content with an error/status message.
- Make existing views "failable".
- Some changes in Conflicts view.