Minor non-functional fixes:

Those are minor, non-functional changes I've made during getting familiar with Isis codebase.

- javadoc
- constants
- Log the exception stack trace for better diagnostics.
- Java 7 improvements
- Use JavaScript event names without the 'on' prefix to avoid the warning from Wicket's AjaxEventBehavior
- add TODOs for potential problems
- remove casting where not needed