Made interactive debugging more convenient

`bootstrap.php.cache` and classes cache can really get in the way with interactive debugging sessions.

This changes the front controller so that when debugging, if the \ XDEBUG_SESSION` cookie is set, the cache files above won't be loaded, so that the actual, original files can be stepped through.

### Points to sort out
- [ ] What about other debugging methods / extensions ? Different cookie ?
- [ ] Does it cover all scenarios (command line / web)