WinRT: preliminary support for building against the Windows Phone 8.1 SDK

This is a step towards supporting "Universal" Windows apps, when building for
Windows Phone.  SDL can now build against the Windows Phone 8.1 SDK, and apps
linked to it can run, however further work and testing is required as some
previously Phone-only code appears to no longer be applicable for
Windows Phone 8.1.  The Windows 8.1 code paths does seem to be preferable, but
should probably be tested and updated on a finer-grained basis.

If in doubt, use the Windows Phone 8.0 projects for now, as located in
VisualC-WinRT/WinPhone80_VS2012/

TODO:
- look at any Windows Phone specific code paths in SDL, and see if Phone 8.1
should use the Windows Phone code path(s), or the Windows 8.x or 8.1 paths
WinRT: suppressed an unused param warning when building for Windows Phone 8.1