Start to add supporting code to windsound and macsound. The latter
remains commented out because I haven't had a chance to try
it on macOS yet.
In order to test it out, I added two more stock sounds:
sa2_xplevelup and sa2_xpleveldown.
rename sys/winnt to sys/windows
move vs (visual studio) folder out of win/win32 and into sys/windows
rename include/ntconf.h to include/windconf.h
rename winnt.c to windsys.c
place visual studio projects into individual subfolders.
This will hopefully resolve GitHub issue #484 as well.
