Update to 2.8.0~pre5

Notable changes:

- Text editor component split off into `QProgEdit`, which should be installed separately: https://github.com/smathot/QProgEdit
- Multiprocessing runner now appears works transparently, but please double-check `multiprocess_runner.py` and `process.py`.

A pre-release package is now also available:

- <http://files.cogsci.nl/software/opensesame/pre-releases/>