Various fixes, mostly to EyeLink functionality

Today I did some work on the EyeLink functionality, some of which was broken due to all the restructuring. Also, `libeyelink.py` seemed to be based on an fairly old version of the `opensesame_eyelink_plugins`, so I migrated some of the newer functionality.

This is still quite early days. I intend to work a bit more on the EyeLink functionality in combination with the OpenSesame plug-ins later this week. So expect another pull request!