Remove eye_movement detector and Windows C++ docs

[pfaion]

I noticed that the only thing we need visual studio for on Windows was nslr, which did not have prebuilt wheels. Since we deprecated the eye_movement detector, we could actually get rid of nslr.

• removed all eye_movement code and replaced cross-module used code
• removed nslr as dependency for all platforms
• cleaned up Windows docs:
  • we don't need a hard-coded C:\work anymore!
  • we don't need Visual Studio
  • we don't need the developer command prompt, simple cmd.exe is enough
  • we don't need OpenCV binaries anymore in pupil_external
  • I removed the notes on Windows paths, since users don't need to adjust paths anymore
  • I also removed old docs for installing optimization_calibration and the therefore needed ceres install instructions