audio: Support 16bit sound source for sound mixer

We have made the sound engine use 16bit buffer to blend multiple
sound sources before the VCS was started, but haven't made sound
sources themselves.  The sound mixing buffer had a higher priority
because further sound sources could yield very little amplitude at
the listener's position.  But now that we acquired high quality sound
sources, we also want to make buffers for them have higher resolution.

8bit sound sources are still supported, although that backward
compatibility rarely helps anything.

Ogg Vorbis files are always decoded into 16bit buffer, while Microsoft
WAV files reserve their sample bit depth in file format.
The raw WAV file can have 8bit PCM data, which have no benefit to
expand to 16bit.

Stereo sounds are still not permitted.
audio: Delete apparently unused funcion in wavemixer.cpp
audio: Add debug window to display waveform being played back

This functionality is disabled by default, but can be activated
by setting g_debug_sound console variable to 1 in autoexec.cfg.
audio: Rename wavsound.h to playSound.h for better indication

Corresponding source wavsound.cpp is also renamed to playSound.cpp.

We cannot really tell the difference between wavemixer.h by the name.
audio: Delete apparently unused functions in playSound.cpp

The functions may be some test functionality for other audio interfaces,
such as DirectSound and MCI device API, but I don't really remember.
They are deleted for the time being to clarify the source's intention.
audio: Rename playWave*() to playSound*() for better indication

The functions are not only capable of playing Microsoft WAVE file
but also Ogg Vorbis files, so the old names are not appropriate.
audio: Add some Doxygen comments to playSound.h
audio: Make playSound() be accessible from Squirrel scripts