Replace OrbiterSound with XRSound

[ethhics]

This PR is NOT complete! Don't merge yet.

Closes #474 .

I want to provide a starting point for y'all before this just sits on my hard drive. Feel free to commit to this branch as needed to finish the implementation.

What is implemented

• src_sys/soundlib has a complete implementation of XRSound in a way that should require minimal modification of any other code.
• Build files have been changed to target XRSound.

Still to be done

• Vessels load and initiate connection to XRSound.dll before XRSound.dll is loaded. This results in nothing being configured from a sound perspective. It appears that the vessels' soundlib code will need to be shifted around so that nothing happens before clbkPostCreation.
• Someone more intimately familiar with the project should do a runthrough to make sure that all of the sounds that are supposed to play, play.
• It would be "The Right Thing" to tear down soundlib and everything related to sounds playing and rewrite it in the object-oriented style of XRSound and take advantage of features like >60 samples. I'm unfamiliar with this project's code philosophy, so if that's more of the approach y'all'd like, just close this PR and I'll devise a strategy for a more thorough codebase surgery.

[indy91]

Thanks for working on this. Looks like a big project. I can't contribute much at the moment, I'm too deep into working on some RTCC MFD stuff. How about you continue on the path you are right now with this PR and try to get some things working, I think it definitely is on the way to be able to be merged at some point. If you want to talk about some of the details or have any questions concerning the NASSP code, feel free to stop by in our IRC channel (#nassp on freenode), that's where we usually hang out.

[ThymoNL]

This is still on the roadmap and it would be nice to have for NASSP 8, this does however need rework to be compatible with the current state of NASSP and it's probably easier to redo these changes from the latest state.