Added FreeTrackClient.dll and adjusted .gitignore

Before it wouldn't build correctly unless you copied it manually, but this didn't break the rest of the build. Because the file is free to redistribute, I decided to just include it in the repo rather than asking the user to copy it manually in the readme.

I also added a gitignore for libOVR, as that *isn't* free to redistribute.

I hope this isn't troublesome to merge, it seems that I've changed some of the same files as https://github.com/cybereality/Perception/pull/20.