Bump to 2.8.0~pre15
Various improvements to psycho canvas back-end
- Add `psychopy_suppress_warnings` options
- Override `set_font()` to register fonts from the file pool (#217)
- Minor code clean-up
Fix `total_response_time` in `item.set_response()`
Add gamma option to psycho backend
Merge remote-tracking branch 'origin/playground' into playground

Conflicts:
	openexp/_canvas/psycho.py
update debian/changelog
Set subject_parity in Android runtime (closes #221)
- Clean up Android runtime a bit
- Bump to 2.8.0~pre16
Set `multiprocess` as default runner
Add 'Why is this important?' link to back-ends combobox
Prepare for 2.8.0 release
Update setup scripts