Add general-purpose LSL interface

[cstawarz]

"LSL" meaning Lab Streaming Layer.

The core LSL libary is written in C++, which is helpful.

[cstawarz]

This issue has been mentioned on MWorks Support. There might be relevant details there:

https://mworks.discourse.group/t/about-interfacing-lsl-protocol-pupil-labs-eye-tracker-with-mworks/1073/8

[cstawarz]

Mostly done as of commit mworks/mworks@096b6d3. Still needs docs and tests.