WinRT: added project files for Windows 8.1 + MSVC 2013

This commit also includes some base-level support for Universal apps.

TODO:
- add support for Windows Phone 8.1
- add support for satellite libs: SDL_image, SDL_ttf, and SDL_mixer

--HG--
extra : rebase_source : cde445519dd05c34d1cfc25b5c2f0734cf960adf
Fixed a linker error when building SDL/WinRT

The Win32 APIs, VerifyVersionInfoW and VerSetConditionMask, are not currently
available for use in WinRT apps.  This change primarily #if[n]defs-out some
calls to them.