Re-add private header files to VC2012 project file

This is the same issue as 56f2e01 (I should have done this there).
We again add private headers to project file for better navigation.
audio: Import sound mixing library from clib

We want to convert the library to C++ and also customize it for our
project's need, so we copied it instead of tweaking existing library.
Another important reason is that exporting functions as DLL symbols
in a static library is counter-intuitive in Visual C++.  It's not
impossible, but we need to specify all functions to be exported in
a .def file.  If it comes to class methods, we need to write down
decorated names.

The sound mixing is performed in a dedicated thread so that the main
thread won't block the sound and vice versa.

I think there are bugs regarding positioning sound source in 3-D
space, which should be fixed with extensive tests.
surface: Add Tank gun shoot sound for demonstrating sound engine

The shoot sound is recorded in real tactical exercise, so it's
copyright-clean.
Erased unnecessary header includes from Sceptor.h and Sceptor.cpp

The long history of Sceptor make it having many include files, including
not really necessary ones.
Add Sceptor gun sound

This is another demonstration of sound engine besides surface module's
Tank.

The sound is recorded in firing exercise, so it's copyright-clean.