WinRT: fixed an invalid comment in SDL_GetPrefPath() (for WinRT)

WinRT: allowed 'roaming' and 'temp' folder paths to be retrieved on WinPhone 8.1 Windows Phone 8.0 either did not define, or provide access to, a 'RoamingFolder' or 'TemporaryFolder' for apps to use. Windows 8.0 and 8.1 do, as does Windows Phone 8.1. This change allows SDL-based Windows Phone 8.1 apps to access these folders, via either the SDL_WinRTGetFSPathUNICODE() or SDL_WinRTGetFSPathUTF8() functions. SDL_GetPrefPath(), which on WinRT, is based on SDL_WinRTGetFSPathUTF8(), will continue to return the app's 'local' folder, despite Windows 8.x counterpart apps using the 'roaming' folder, in order to preserve compatibility when 8.0-based Phone apps upgrade to 8.1-based Phone apps.

Fixed typo in header file documentation comment.

Fixed bug 2726 - WinRT touches not setting 'which' field in virtual mouse events This patch makes sure that any SDL_MOUSEBUTTONDOWN, SDL_MOUSEBUTTONUP, and SDL_MOUSEMOTION events, as triggered by a touch event in a WinRT app, set the event's 'which' field to SDL_TOUCH_MOUSEID. Previously, this was getting set to the same value as events from a real mouse, '0'. Thanks to Diego for providing information on this bug, and to Tamas Hamor for sending over a patch!

Corrected header file documentation comment.

WinRT: updated README-winrt.md to detail Visual Studio 2013 Community support

WinRT: made SDL_GetPlatform() return "WinRT" and not 'Unknown' Thanks to Eric Wing for the heads-up on this!

WinRT: build fix for Visual C++ 2013 Update 4 Visual C++ 2013 Update 4 re-introduced the Sleep() function to WinRT apps (for code that targets Windows 8.1 and Windows Phone 8.1). This led to a build error, as SDL was defining it's own Sleep() function (to make up for the lack of a public Sleep() function). The fix makes sure that SDL's custom Sleep() function is only used when Windows' Sleep() is not available. Many thanks go out to Sergiu Marian Gaina for the fix!

Fixed gcc-fat.sh failing to build Objective-C sources correctly. This would fail if the compiler was given a .m file _and_ no "-o" option; it would fail to automatically calculate the correct .o filename in this situation. Thanks to Ethan Lee for the help tracking this one down!

WinRT: fixed multi-touch bug whereby 2nd, 3rd, etc. fingers weren't moving Only the first-pressed finger wpuld get reported as having moved (via SDL's touch APIs). Subsequently pressed fingers wouldn't report as being moved, even though the OS was reporting (to SDL) that they had moved.