Add Gaze Input method (Eyetracking via Webcam)

[klues]

Add gaze input as an alternative input method. @sabicalija already worked quite a bit on it. Just found WebLinger.js, maybe it's interesting.

[sabicalija]

Thank you for the link! I'll try it out definitely. The documentation looks already better than the docs of some alternatives.

Btw.. should we move the branch from the fork to this repo (https://github.com/sabicalija/AsTeRICS-Grid/tree/alija/feature/eye-tracking)?

[klues]

I think he integrated some other libraries in this one. You don't necessarily have to move the branch - for the moment it should be enough to have linked it here in the issue.