XAudio2 2.8's header file, xaudio2.h, doesn't compile in plain C code for WinRT
apps, not automatically at least. Initially, this file was adapted to compile
as C++, however these changes are now deprecated in favor of some preprocessor
based hacks that should get xaudio2.h to compile (while making sure XAudio2
still works).
- moved SDL_WinRTApp.* from src/video/windowsrt/ to src/core/winrt/, and renamed them to SDL_winrtapp.* (to mimick case-sensitivity used elsewhere in SDL)
- renamed all "windowsrt" directories (in src) to "winrt", as the shorthand name is used more often (and, IMO, "WinRT" != "Windows RT", not entirely at least)
